There are only two things you can replace on the Vanos (single or double). One is the seals, which are all bad due to the material issue. These must be replaced. The second is the bearings. These only need to be replaced if the Vanos is rattling, usually on cold startup. This is a bit more work. Another option is the Dr. Vanos, which is a complete rebuilt Vanos. It's just a straight swap. That's $250, after core charge, but saves about 2 hrs since you don't need to replace the seals or the bearings.
$400 sounds a bit low for a Vanos seal replacement. It takes me about 5-6+ hrs to do this job and I have done it multiple times. I cannot see anyone doing it in under 3 hrs, given the amount of parts that need to be removed. The parts (seals $60 + VCG & grommets $45) are over $100. That leaves less than $300 for labor. Here in NE, that'll buy you about 2 hrs of work. So it seems low to me.
Honestly, I would recommend doing the Vanos job yourself. Yes, it will take you 6 hrs but then you get to see the inside of your engine. For folks that don't get to do these kinds of things, it is kinda neat looking inside that I6. The only special tools required are the fan clutch tool and 32mm wrnech (~$30). You should have everything else. While the $250 savings is nice, the satisfaction is immense.
This is my favorite DIY as the benefits are usually very noticeable. And the Beisan instructions are amazingly goof. A very high ROI!