It depends on what specific features you are looking for. Although not officially tested and not supported, the RPAD can use the CMA as a gatekeeper. This would mimic the functions of the VBP-E.
Where we get into some feature difficulties is setting up the Access Proxy which mimics the VBP-ST functionality. Although the basic design is the same, 5222 for presense, 443 for presence, and 389 for authenication is roughly the same, there are some other difficulties present. The provision profiles have changed from CMA to the RPRM. Also the CMA requires the the VBP-ST enter both internal and external IP addresses which may not be present in the RPAD with a single IP address.
Bottomline: I would recommend migrating from the CMA to the DMA/RPRM in order to fully support al features you are looking for.