how do i backup a vm with rdms with vdp?
Are the RDMs in physical compatibility mode?
If thats the case unfortunately vdp will not back them up.
For those virtuals traditional backup methods should be used.
can i change from physical compatibility to virtual?
Yes you can change from physical to virtual, but you must remove the RDM from your VM (i prefere to use "Remove from virtual machine and delete files from disk") and than you must add it again with using the "Virtual" compatibility mode. You don't touch the RAW device itself, so there is no risk with the "remove from disk" option. After that the VDP backup should work with the RAW device, but if you read the documentation it is not supportet.
Paul