Verified in 5.6.1.0. Tried with this code:
vm_id = '1234'
query = if vm_id
           $evm.vmdb(:vm).where("vendor = ? and id = ?", "redhat", vm_id)
        else
           $evm.vmdb(:vm).where("vendor = ?", "redhat")
        end
query.first

and it passed.

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://rhn.redhat.com/errata/RHSA-2016-1634.html