VX210 is 8Ohm and 250 Watts, 4.5XL is 400W and 8 Ohms, You'd probably end up with a 200W cab,a s (i think) the cones in the VX are 125W each, and the XL cones are 100W each.
Depending on how they're wired inside the cabinet your impedance may drop to 4 Ohms, which might be an issue if your amp is delivering 250W at 8 Ohms (to match your current cab).
If your head will go down to 4Ohms, then you'll be chucking out close to 400W (max) into a 200W cab - net result is two more blown cones...