I've been editing a six-episode, 53-minute-per episode TV series to run on the community access cable channel.

Their standards given to me: -18dB average, no peaks over -15 or valleys under -25.

I've done plenty of audio-only mixing over the years, and audio for video mixing for youtube and the like. It's always been 0db is the max, otherwise loudness to taste.

So, I'm a little stumped as to how to get my mix into these parameters.

I'm using Sony Vegas. I've been building and mixing the show with the master at 0, not aiming for anything other than "sounds good" and "doesn't clip" by just using levels and some conservative compression and plugins.

What I'm wondering is ... 
- ought I re-mix the whole thing and aim for a lower peak, or just keep mixing with 0dB as my ceiling and bring it all down and into the specified window of peaks and valleys afterward before delivery?
- if mixing normally and handling the specs after the fact is the better strategy -- where to go from there? Drop the master to -15 and compress from the bottom up? Maybe mix the whole thing down to one long audio file and take it into a DAW, put Ozone on it, as though I'm mastering it, and lay the mastered audio back in?

I feel like the answer is staring me in the face and I'm just psyching myself out about this. As though it's as simple as "go into Ozone, use preset x, threshold at -25, ratio at 4:1, limiter at -15, EQ to taste." Thoughts?