I am wondering why you don't give up Procmail? Dovecot has Sieve and you install this package with Virtualmin.
Well, a lot of Virtualmin's email features are tightly integrated with procmail... the way it handles Spam and Virus scanning, for example. Usermin also comes with tools for building and managing procmail rules.
Sieve was fairly new when Virtualmin got started -- and at this point, switching it for a different tool is a rather large project.
While procmail does work well for our needs, that's not to say we'll use procmail forever... newer tools such as sieve are being actively maintained, and have some pretty neat features.
Actually, there's quite a few Dovecot features in general that are becoming more widely available, that we'd really like to put to use.
After we finish up some other projects we're currently working on, and we begin looking into some of those new Dovecot features, we'll be sure to evaluate whether procmail is still the right choice, and what it would take to switch to sieve or a similar tool.
Also, I thought I'd mention that some folks who wanted Sieve came up with a way to deliver emails using Dovecot's LDA, but through Procmail... that is an interesting way to switch to Sieve, even while still continuing to use Procmail:
That came up in another thread here recently.