Here's how I see this playing out, since my brain is a bit more operational right now.
The vanguard are either attacking the HQ or not. If they are, we are probably fucked and never going to be able to take it, because I doubt we can kill them before they can reinforce themselves. If they didn't know about the surface based defenses previously, (therefore causing them to attack the HQ and secure it since its dangerous) then they would once they had taken it over due to seeing them. I say this because defenses that can damage them probably aren't small or disguised to not look like enourmous weapon systems. Maybe hidden from orbit, but they will be in the base at that point.
If the vanguard have anything to do with the HQ we will be truly and honestly fucked and attacking would be suicide as a result. We would be best off hiding and going Vietcong as mentioned, or surrendering.
However, if the vanguard don't know about the HQ and haven't attacked it yet, then we have a golden fucking opportunity. If they don't know, then the slinkar will be pulling out everyone except a skeleton crew in a desperate attempt to stop the vanguard elsewhere with everything they have. The place will probably be almost totally unprotected compared to what we would have had to deal with earlier.
I think we have to drop regardless, because alien politicians, but we need to scout out the HQ. If there are any signs of vanguard presence flee then become guerrilla fighters (however ye spell that). If not then we just attack the fucking place based on what the situation looks like when we get boots on the ground. This needs to happen as soon as possible because its obvious the vanguard move quickly.
They crushed a decades long stalemate in space, I doubt it will take them much longer to make headway on the surface. We aren't going to have mountains of time to use the defenses if we manage to take the place. We are facing a massively superior foe and the path to victory is narrow. That being said, LETS DO DIS.