I can only find one way in to their compound, by picking the door by the vendatron! Problem is the door is red, so everyone becomes hostile! Is this supposed to happen or am I missing something? Do I just slaughter them and "borrow" their stuff or is there another way in? Are they aligned with another faction? I don't wear faction clothes! What am I missing??

Yes, it is there so you don't go into it, they are a private industry.

There's a quest (You Can Depend On Me) where you break in and steal their manufacturing specifications and give them to the Crimson Caravan (you can turn that task down without penalty if you so choose). But the only way in is to pick the lock on the gate and sneak in. You'll obviously want a Stealth Boy or two to do this.