I just have an RB260GS. I configured a trunk with 4 vlans (port1) and the the other ports as untagged one vlan after the other
If I set the System >Address Acquisition > DHCP with fallback, i did not get IP. I am not surprised as I am sure the Switch did not know via which VLAN have to send the DHCP request and get the IP. I have check the Manual, but I did not found any option to set this. If I set the expected IP all is fine. The Device did not asked the Mask and the GW but both is working. Somehow it find out. 
Can I force the DHCP request to a specific VLAN? (SWOS 2.11)
I don’t recall that you can. I have always set static IPs on network elements.
k6ccc, I fully agree, but in test environment I prefer the dhcp. When I reset a routerbord I always know what IP will be assigned.
In this case generate an intersting question. 