I'm a big fan of Battlefield Vietnam these days, but you will get more value out of Battlefield 1942. I never really got into too much of the regular gameplay of BF1942...I totally agree that the foot soldier is at a major disadvantage, but it is not so in Desert Combat. In Desert Combat there are several maps that are dedicated to foot soldier fighting and it is much easier being a foot soldier and getting some good kills.
Joint Ops is okay, but the gameplay has a very awkward feel in comparison to Battlefield in my opinion. Some people seem to love it though...so I'm sure if I kept at it, I'd enjoy it more...but it just doesn't peek my interest enough.
I think we should see some really cool mods coming out for Battlefield Vietnam...and even without the Mods, I love the regular gameplay. You have the same "foliage" that you get out of Joint Ops, with the superior gameplay/vehicle phsics of BF. The helicopters are a blast to use and with the newest patch, the sides are pretty evenly balanced (well so it seems with my limited playing w/ the new patch...just released yesterday). Also, I love the atmosphere of the game that makes you really feel like you are in Vietnam...from the music, to the map design and graphics. A lot of fun. I will agree though that you will need a decent machine for a fun gaming experience. If you don't have a decent machine (with plenty of memory), just get BF1942.