I plugged in a Windows 10 host and set the NIC to pull an address from the default LAN, that worked without issue; I was able to ping the 192.168.1.1 default gateway. When I manually set the host's IP to the 192.168.15.x subnet (192.168.15.11 in this case), I'm unable to ping the 192.168.15.1 gateway. I did run a packet capture while doing all of this and there is activity on the VLAN 1 NIC but there was no activity captured on VLAN 15.
You missed a very important part, you have to tag your pc's nic with 15 in order to plug directly into opnsense.Without tagging it, it will only ever get your LAN traffic.
No, that's just setting an IP address, has nothing to do with a vlan.What kind of nic is in the pc?As I said earlier, if it's an intel nic, download their ProSet driver. It allows tagging.If it's not an intel, you'll have to find out from the manufacturer.Go into the nic's properties, then click configure.Do you see a vlan tab?