How do you configure PXE to work across VLANs?
Make sure that all network routers and layer 3 devices have an IPHelper address configured to relay client bootup communications to the PXE server.
The PXE client will also need to be able to speak to the DHCP server; therefore an IPHelper address also needs to be configured to relay any 'bootp' communications to the DHCP server as well.
If you are using ACLs (Access Control Lists) to specify what type of traffic can communicate between VLANs, make sure that communications on ports 67, 68, and 69 are allowed.
If you are using Multicast for the TFTP bootfile transfer, make sure that ports 69, 1758, and 1759 are open on any ACLs that might be in place.
Since you will more than likely be connecting to managed switches, make sure that "portfast" (or the equivalent of "portfast" depending on hardware) is enabled on the client port so that the port can start forwarding packets immediately upon bootup.