Looks like I'll be upgrading then. 2.2 is the highest official version supported. Is there anything that I should look out for when doing a custom build? It looks like CyanogenMod is a popular choice.
I've run loads of custom ROMs. Some are good, some are bad. Get an old/and or small SD card, backup your current ROM, and then try a couple out. I have no experience of your phone, but on many it's very straight forward to install them. You need to do you research though, just to make sure there's nothing involved that could potentially brick it.
I've recently moved to a CyanogenMod ROM and I really like it. It's not the "official" version as that one was too old (I wanted Jelly Bean), but it is a "stable" version and I've not had any problems. I think it's a safe option. In the past I have used ROMs based on CM, and others that are nothing to do with it, all with vary degrees of success. Have a play.
This would be a good place to start http://alchemistar.blogspot.co.uk/2010/11/samsung-vibrant-custom-rom-list-1110.html