Hi,
The 4s really isnt designed as an upgrade to the 4 its for people coming off the 3gs if anything. so waiting is a great idea, but dont expect mircales off the next iphone there probably wont be any radical hardware improvements , just software tweaks and "hopefully" a bigger screen. but certainly it has to be better than the 4s for you!
Your contract with Vodafone 12/18/24m ect is a minimum contract period once you are outside of that you are free to upgrade or cancel with 30 days notice or request a pac code to take your number to another network.
If you do nothing your contract rolls on a monthly rolling basis untill you choose one of these options.
If you went on a high tariff to subsidise the iphone that has more minutes than you actually need, you may wish to ask for the plan to be lowered (without recomiting i.e 30 day sim only) then put it back again when you upgrade.
Fingers crossed for June/July but if it is Oct/Nov that could save you quite a bit between now and then 
