Analog Devices sells eval boards for some linear vector
modulators. I used a rack of four, to make the phase
field for a digital vector modulator I was designing. You
would need a 90 degree phase split somewhere in the
system because they want I and Q inputs (using baluns
I believe, to split phase fromt here).
It would surprise me if there weren't something more
elegant out there now, with a single-phase input being
a real nicety.