Giving Vellos outfits

and failing

I'm trying to make a plugin so I can give a vellos ship outfits that I have created.

I got to a point where I created the outfit and I created a mission. The availability was set at bit 420. I got it so the vellos ship had to land on a specific planet to get the mission. I wanted the mission to just happen then, but I would take off and and it would happen in space. That was fine, I wasn't picky. However, when I landed the mission started over again. My first guess is that bit 420 was still available.

I could do the 'not' bit (!b420) "on mission success", but I am affraid to do this since bit 420 is used in the story line. I have looked at the walkthrough and as far as I know it is only used once.

If I 'not' the bit will that screw anything up?
Is there another way? Also I am not good with plugin creation so I would rather not play with crons if that were another way.

Thanks

The best solution would be to find a bit that's not used by anything else and have the mission require that bit not be set in order to activate. Then simply have the mission activate that bit upon completion.