It could be that your devices are not exposing the correct MIBs to the SNMP response in order for SolarWinds to gather the data. Take a look at this thread:
About thre quarters of the way down zizi mentions the specific MIBs that Orion uses to gather VLAN information:
vtpVlanTable = 18.104.22.168.22.214.171.124.126.96.36.199
vmMembershipSummaryTable = 188.8.131.52.184.108.40.206.220.127.116.11
dot1qVlanIndex = 18.104.22.168.22.214.171.124.126.96.36.199.2
Are these MBS returned if you do a SNMP walk to the devices? If not you may need to enable these in your device config.