1) Yes, the VTx technogolgy enables a 64 bit guest OS to run directly on the real CPU (only 64 bit processors have VTx). With VTx, you can run a 64 bit guest on a 32 bit host, because the VTx gives the guest access to the 64 bit CPU via VTx. When VTx is turned off in the bios, VMware and VirtualBox must emulate a processor (with a a significant performace penalty), and a 32-bit host can only emulate a 32 bit CPU, so the 64 bit OS does not work. I think, but not 100% sure, that even a 64 bit host OS can only emulate a 32-bit processor without VTx, but its been a while since I tried that, so I may be wrong.
2) I mostly use KVM now for my virtuals, so I am not sure on this either, but I would say the enable/disabling of VTx inside VMWare and VBox, on a per VM basis, would also mean only a 32-bit guest can run in a virtual with VTx disabled. If a 64 bit guest can run on a 32 bit host, with VTx turned off at the hypervisor (ie VMware/VBox interface), I would question if the option is really disabling VTx.