Make up your own packs from some decent AA's
As you said, the only reason you need to change them is that your current ones are so old - stick some new AA's in and use your current charger - job done!
If you do go down the lipo route i'd check out the difference in weight between them and AA's - if there's a lot of difference it will affect the feel and balance, and to me, that's what makes the Vantage soooooo good.
