oyebaba,
first drain the vessel before flooding it, size the demister to remove the airborne droplets and contaminants without choking the flow, use an inlet distributor for the initial gas and liquid separation. The levels are determined by the inlet flow and therefore by the size of your vessels. I believe that you already know all this. If you don't do all the above, your gas flow will carry over to much droplets and the demister will be flooded and the pressure drop will force the droplets to be dispersed downstream of the demister. If your vessel is not tall enough, to allow separation height, you won't achieve the desired separation. The design of these vessels is described in great detail, including the free downloads on the internet. As you said, the critical velocities allows you to size the vessel, the rest is your mechanical engineers job to make it happen.
All this is good, however, every fabricator of these separators, knock-out vessels, drums have their own guaranteed software to design and produce guaranteed process vessels. Do you really think you'll out-perform their experience and design a better separator (minus guarantees) than them?
Anyway, good luck to you.
cheers,
gr2vessels