I'm of the opinion that quality matters more than quantity. They could have 10 vehicle options, but it wouldn't matter if only 2 of them were worth taking.
As it stands now, it looks like the vehicles that Sisters have available are a good fit for their army. I'm sure that SOB players would love to see the Land Raider as an option, but who wouldn't?
The question to ask is not "Do the Sisters of Battle need more vehicles". Rather, the question that should be asked is, "Is there a short coming in the Sisters of Battle army list that can be filled with a new vehicle option?". Personally, I don't really think there is. The Sisters do lack long range fire support, with the only option being the Exorcist, but this roll might be better filled by making Missle Launchers an option for troops rather than making a new vehicle option. Then again, long range isn't what the Sisters of Battle are about. You would probably be better off investing points in more Rhinos rather than optional weapons like missle launchers.
If you really feel like you need more vehicle options there is always VDR. You just have to find opponents who will agree to those optional rules.