It sounds like you are on the right track. So port 25 faces the router? Which port faces the ESXi server? That port will also need to have VLAN 10 and 20 set to be tagged.
Hello everyone. I have a Dell PowerConnect 2848.  My Router is a Netgear SRX5308. In the router, I have created multiple VLANs (VLAN ID 10 and 20) and want to pass that over to the ESXi server. If I connect the ESXi server directly to the router, everything works as expected. My VMs are picking up correct VLAN based on the ESXi settings. I need the 2848 in between as I need to add more devices and additional servers that have specific VLAN.  Currently, I am using port 25 for the 2848 switch.  I have put the switch in Managed mode.  In Switch -> VLAN -> VLAN Membership, I created VLAN ID 10 and 20.  I select VLAN 10, and put T on port 25. I select VLAN 20, and put T on port 25.  (I also tried putting a U on them as well, just to try, but did not work) But my the VM are not able to reach DHCP on the router.  Spanning Tree is enabled. I am obviously missing something...  I have already spent last week banging my head on this but have not been able to pass the traffic along. Please help!!!!  
PowerConnect 2848 - Multiple VLAN on 1 port not working