• If vamps have 'induced dormancy' whenever they're cut off from air, how fast does it work?
• If he fell in the lake, would he immediately go dormant as soon as the water went over his head, or would he be able to swim back to the surface and take a breath (i.e. does he get as long as a human to hold his breath)?
• If I submerge the bottom half of a vamp do his legs go dormant while his upper body stays mobile?
• Does it matter what kind of air is available? If I put a vamp in a sealed container does he eventually use up all the oxygen in the container?
• And if he does use up the oxygen and the air goes 'bad' can we assume that he then turns dormant?
• If halon extinguishers go off (which flood an area with inert gas, pushing out any oxygen) would he immediately pass out? It's still technically 'air', but probably isn't considered 'good air'.

If dormancy is immediate then something like splash grenades with a liquid sealant or freeze foam might be the easiest way to incapacitate one.

Pretty self-explanatory, I think. It has nothing to do with touching water and takes a few minutes to take hold (ie: A vampire is actually able to act longer underwater than most regular people).

Yes, the vampire can hold his breath as long as a human can. When a human would suffocate and die, a vampire goes dormant, waiting for a renewed supply of air.

If I submerge your legs in a swimming pool with your upper half above water, do your legs go dormant?

No. He remains fully mobile (though wet).

It's all about the oxygen, baby. In order: Yes, it matters. Yes, he runs out of oxygen. Yes, he goes dormant. Yes, halon would do the same thing.

Just keep in mind that (Essence) Minutes can be up to 12 minutes for a vampire human/metahuman...

If you are planning on taking on a vamp by suffocation, you'll need to wait at least that long before attempting anything. Then again, watch out if he has the internal air tank cyberware. Then all bets are off and he's likely waiting to ambush you.