When I bought my Supermicro X9SCA-F (also C204) and added a PCI-E graphics card, it also disabled the on-board video. There was no BIOS setting to override. Updating the BIOS to the most recent that was only 6 days old did offer that option.
So updating the BIOS is something I'd definitely advice.
DD-WRT can handle VLAN's, so a new switch isn't necessary. ESXi has no problems with multiple VLAN's on a single NIC, just configure the VLAN ID on the virtual network the VLAN's in DD-WRT. Then add routing rules to allow traffic between them.
(I tried this on a WRT54g, but DD-WRT became...