I would have to concur with other posters here. PTA is just too specialized, IMHO, even for an RN to work in that exact role. I'm not saying that an LVN or an RN can't work with patients in a rehab environment, it's just that to work as a PTA, the education is quite specific and the learning curve would be intensely steep. Probably about the only other professional that could fairly easily step into the PTA role without much (if any) additional education is the Athletic Trainer. One of the biggest differences between PT and ATC is the client population. An ATC is NOT a PT and a PT is not an ATC. While there's tremendous overlap between the two fields (and can be a LOT of professional friction), they're different fields. It is NOT uncommon, however, for an ATC to attain the additional education to become a PT.
RN and LVN programs just don't cover the required information for PTA. It takes a 2 year program to get to PTA... think an RN program covers that much material for rehab even with all the other education that must be covered for RN licensure? LVN coursework covers even less. It's just not possible.
The state where I live grants licensure to PTAs. Therefore, they are called LPTAs (licensed physical therapy assistants) in this state. The PTA is an associates degree program. If the LVN does not have the LPTA license, (s)he cannot legally work as a PTA here.
However, LVNs are able to work as rehab techs. A rehab tech can learn the duties of the position through on-the-job training.