I have a trek 720 with canti posts, can I switch over v brakes from another bike?

No difference on the mounting posts for canti and v's. Exception would be u-brakes and roller cam type cantis.

Just to clarify.

There's no difference in the type or position of the posts, but there is a difference in the cable response (leverage) between canti's and "V" brakes. After switching you may find it necessary to replace the levers also.

There can also be an issue with the horizontal post-to-rim distance. Older canti's were fairly insensitive to that because the shoe was on a post and could be moved closer, but most V brakes are much more sensitive and if the distance is off, for example if you're using a narrower rim than was originally spec'd. the performance of the brake may suffer.

So yes, you can swap, but be aware of the total implications.

By swapping the thick and thin washers on the v brakes around (and possibly adding a thin washer on the inside in the case of very thin rims) you can compensate for fairly substantial differences in frames and rims. I have converted many many many bikes form canti to v (although most of them were ~15 years ago) and never once had sub-standard perfromance after they were set up.

I often found cable routing was more complicated than pad positioning when converting.

FBinNY raises some good points. I'm glad I put cantilevers on my Cross Check. I could have used V brakes. The only reason I didn't was that I consider V brakes to be ugly. Not very rational, but I'm honest. Now I have another rationale I can give.