This is what Layer 3 is for. You need to open the relevant port between vlans (e.g. TCP 443 for https) on the firewall. I think its UDP 1900 but may vary by appliance.

I’d also allow multicast, ICMP (ping) and DNS between your vlans as a minimum depending on what they’re used for.

It is just water. When the engine starts the cold exhaust heats up creating condensation which drips off as the exhaust fumes are expelled.

It “can” be oil, but you’d need to be pretty unlucky, and need a new engine.