Small Scale Military Headquarters

Well I just bought a BF-109 Winter Version from [CAT]cplslade and i'm looking at adding decals (the non-P.C. ones) to it along with some of my other German A/C's, I've never bothered with them, but now looking to do a little detail. My question is, is there anyway I should go about doing this? Normally on my models I clear them, warm up some vinegar and soak the decals in them, then place the decals on, maybe brush a little more vinegar on to make sure they fit in the nooks and crannys, then apply a matte coat. Any suggestions on this since the 21C/Ultimate Soldier ones are pre-painted and all?

I've always used water as opposed to vinegar, then perhaps brushed on a little MicroSol to "melt" the decal into any cracks or panel lines. Otherwise, my process is the same as yours, matte coat and all.

I've never had a problem putting decals onto the pre-painted surfaces of the 21c, BBI, etc. products. They go on just like any model I hand-painted myself, even perhaps a little easier. However, I would only apply a matte coat if you do not intend to remove the decal. The matte coat will seal the decal and if you remove it later, there could be some discoloration on the painted plastic...