Campanolo record power meter crank with veloce crankset Can I use a Stages Campagnolo Record left crank with my Campagnolo veloce crankset,   if the sizes are a match?

Want a power meter of some sorts and it would be far cheaper than getting vectors.
No.

The Veloce crankset (assuming it's a 2011+ version) uses Campy's Power Torque system to attach the left crank arm.

The Centaur crankarm offered by Stages uses Campy's Ultra Torque design, as do all of the Campy crankarms Stages offers.

They are completely different.