you mean finish the main story line? Or finish the entire game?
Fallout NV isn't a sequel. In other words, there won't be any spoilers if that's what you're worried about. It's about like morrowind vs oblivion in that regard. I recommend just playing NV, and then go back to fallout and play it when you want. I prefer to take in the scenery and enjoy the game at a casual pace. Rushing through games kills a lot of the fun, IMO. Anyway, just some food for thought.
There are mods that deal with the level 30 cap (changing it to 40, for example), in case you are interested in that. Just check out fallout3nexus or similar sites. Personally, I like gaining points and leveling, regardless of any real benefits.