Yeah, as soon as the guy selling them realised that the 3G was out he's bumped the price up. Understandable really. Even then, £5 a month for 3GB is a great deal. Even GiffGaff only gives you 500MB for that, 3gb costs £12.50
Order a 3G and take the WiFi back once it's arrived
First things first, use Content Manager and do a Backup to your machine of choice (PC/Mac/PS3). Then whatever happens you have all your saves and stuff. Set your new Vita up and once happy, on the old one go into Settings, PSN, Activation, Game and click deactivate and do the same for Video. That way your PSN account won't be tied to two Vitas.
The advantage you have is that you'll have 2 vitas to make the change, most people doing it will have to do it before swapping them out. At least you can double check everything before you box the old one up.
If you want to see what devices your PSN account is currently tied to go to
https://account.sonyentertainmentnetwork.com/login.action?service-entity=psn