Re: connecting the key ignition to the 12v contactor.
On my Golf the vacuum pump runs straight from a wire which comes from the back of the ignition switch. Use a multimeter to find which one. If you're in any way unsure of whether the wire is capable of carrying the current then I would advise using a simple 12V relay to switch the vacuum pump in and use the feed from the ignition to switch the relay.
Same with the controller but if you need to pre-charger then you'll need to think about contactors?
Hope this helps,
1993 VW Golf Mk2, 9'' ADC, Open Revolt, Trojan 150Ah 12x12V batteries, 2.5KW Elcon charger, motorsport hall effect throttle.