The SRST router does not need to hear, or be in the same VLAN as the CallManager. The only restriction on the SRST router location is that the phone has to know where it is to register to it.
By default, the phones will attempt to contact their default gateway as their SRST router. This is a restriction in earlier versions of CallManager and the IP phone software. In newer versions - 3.2(3) and above, and 3.3 and above, you have a concept of "SRST References" that can be assigned per Device Pool, and you can tell the phone to register to an arbitrary IP address for SRST service. You are no longer forced to have it as your default gateway, but you can continue to design it that way if you like.
