1
 
                                                                     EXHIBIT 5.1
 
             [LETTERHEAD OF BALLARD SPAHR ANDREWS & INGERSOLL, LLP]
 
                                                                    May 13, 1998
 
Vornado Realty Trust
Park 80 West, Plaza II
Saddle Brook, New Jersey 07663
 
     Re:  Registration Statement on Form S-8
        dated May 13, 1998
 
Ladies and Gentlemen:
 
     We have served as Maryland counsel to Vornado Realty Trust, a Maryland real
estate investment trust (the "Company"), in connection with certain matters of
Maryland law arising out of the registration of 4,419,540 common shares of
beneficial interest, $.04 par value per share, of the Company (the "Shares")
covered by the above-referenced Registration Statement (the "Registration
Statement"), under the Securities Act of 1933, as amended (the "1933 Act").
919,540 of the Shares (the "Trust Shares") were issued to The Chase Manhattan
Bank, as Trustee for the irrevocable grantor trust (the "Trust") established
under the Trust Agreement, dated as of December 2, 1996 (the "Trust Agreement"),
between the Company and The Chase Manhattan Bank and 3,500,000 of the Shares
(the "Option Shares") may be issued to Michael D. Fascitelli upon exercise of
options granted by the Company to him pursuant to the Employment Agreement,
dated December 2, 1996 (the "Employment Agreement"), between the Company and Mr.
Fascitelli. Capitalized terms used but not defined herein shall have the
meanings given to them in the Registration Statement.
 
     In connection with our representation of the Company, and as a basis for
the opinion hereinafter set forth, we have examined originals, or copies
certified or otherwise identified to our satisfaction, of the following
documents (hereinafter collectively referred to as the "Documents"):
 
     1.    The Registration Statement, filed with the Securities and Exchange
        Commission (the "Commission"), pursuant to the Securities Act of 1933,
        as amended (the "1933 Act"), and the related form of prospectus in the
        form in which it will be sent or given to employees of the Company in
        accordance with Rule 428(b)(1) under the 1933 Act;
 
     2.    The Amended and Restated Declaration of Trust of the Company, as
        amended (the "Declaration of Trust"), certified as of a recent date by
        the State Department of Assessments and Taxation of Maryland (the
        "SDAT");
 
     3.    The Bylaws of the Company, certified as of a recent date by an
        Executive Vice President of the Company;
 
     4.    Resolutions adopted by the Board of Trustees of the Company relating
        to the issuance and registration of the Shares, certified as of a recent
        date by an Executive Vice President of the Company;
 
     5.    A specimen of the certificate evidencing a common share of beneficial
        interest of the Company, certified as of a recent date by an Executive
        Vice President of the Company;
 
     6.    A certificate of the SDAT as to the good standing of the Company,
        dated as of a recent date;
 
     7.    A certificate executed by Joseph Macnow, an Executive Vice President
        of the Company, dated the date hereof;
 
     8.    The Employment Agreement;
 
     9.    The Trust Agreement; and
 
     10.   Such other documents and matters as we have deemed necessary or
        appropriate to express the opinion set forth in this letter, subject to
        the assumptions, limitations and qualifications stated herein.
   2
 
     In expressing the opinion set forth below, we have assumed, and so far as
is known to us there are no facts inconsistent with, the following:
 
     1.    Each of the parties (other than the Company) executing any of the
        Documents has duly and validly executed and delivered each of the
        Documents to which such party is a signatory, and such party's
        obligations set forth therein are legal, valid and binding and are
        enforceable in accordance with all stated terms.
 
     2.    Each individual executing any of the Documents on behalf of a party
        (other than the Company) is duly authorized to do so.
 
     3.    Each individual executing any of the Documents, whether on behalf of
        such individual or another person, is legally competent to do so.
 
     4.    All Documents submitted to us as originals are authentic. All
        Documents submitted to us as certified or photostatic copies conform to
        the original documents. All signatures on all such Documents are
        genuine. All public records reviewed or relied upon by us or on our
        behalf are true and complete. All statements and information contained
        in the Documents are true and complete. There are no oral or written
        modifications or amendments to the Documents, or waiver of any of the
        provisions of the Documents, by action or omission of the parties or
        otherwise.
 
     5.    The Shares will not be issued in violation of any restriction or
        limitation contained in Section 6.6 of the Declaration of Trust.
 
     The phrase "known to us" is limited to the actual knowledge, without
independent inquiry, of the lawyers at our firm who have performed legal
services in connection with the issuance of this opinion.
 
     Based upon the foregoing, and subject to the assumptions, limitations and
qualifications stated herein, it is our opinion that:
 
     1.    The Company is a real estate investment trust duly formed and
        existing under and by virtue of the laws of the State of Maryland and is
        in good standing with the SDAT.
 
     2.    The Option Shares have been duly authorized for issuance pursuant to
        the Employment Agreement and, when and if issued and delivered against
        payment therefor in the manner described in the Employment Agreement,
        the Registration Statement and the resolutions of the Board of Trustees
        of the Company authorizing their issuance, will be (assuming that the
        sum of (i) all shares of beneficial interest issued as of the date
        hereof, (ii) any shares of beneficial interest issued between the date
        hereof and any date on which the Option Shares are actually issued (not
        including the Option Shares) and (iii) the Option Shares will not exceed
        the total number of shares of beneficial interest that the Company is
        authorized to issue) validly issued, fully paid and nonassessable.
 
     The foregoing opinion is limited to the substantive laws of the State of
Maryland and we do not express any opinion herein concerning any other law. We
express no opinion as to compliance with the securities (or "blue sky") laws or
the real estate syndication laws of the State of Maryland.
 
     We assume no obligation to supplement this opinion if any applicable law
changes after the date hereof or if we become aware of any fact that might
change the opinion expressed herein after the date hereof.
 
     This opinion is being furnished to you for submission to the Securities and
Exchange Commission as an exhibit to the Registration Statement and,
accordingly, may not be relied upon by, quoted in any manner to, or delivered to
any other person or entity (other than Sullivan & Cromwell, counsel to the
Company) without, in each instance, our prior written consent.
   3
 
     We hereby consent to the filing of this opinion as an exhibit to the
Registration Statement and to the use of the name of our firm therein. In giving
this consent, we do not admit that we are within the category of persons whose
consent is required by Section 7 of the 1933 Act.
 
                                    Very truly yours,
 
                                    /s/ BALLARD SPAHR ANDREWS & INGERSOLL LLP
 
                                    --------------------------------------------