I am learning about the VME64 bus. It has only 16-bits of data bus on its P1 connector while no data pins on P2 connector.
Does it means that VME64 is actually a 16-bit bus?
Not sure where you took this information from...
Connector P2 has additional address pins A24...A31 and additional data pins D16...D31.
That is, P1 and P2 together offer 32 address and 32 data lines.
So, is VME64 just a 32bit bus?
No, indeed, it can address and transfer 64bit by multiplexing the available 32+32=64 lines: First it uses all lines to specify an address, then it uses the same 64 lines to transfer data. Of course, this makes sense only when transferring bulk data.