I'm not sure if they could distinguish between data for voice and data for VoLTE. Theoretically, yes, but why make the iinvestment to do it?
Also, you'd have to look at how much data it actually uses. My plan (which I'm sure many have) is 450 minutes a month. VZW to VZW is free, so I probably hit something like 700 minutes or so. Let's just call it 12 hours. One of the more inefficient codecs would use about 1 meg a minute. So we are talking 700megs a month or so for your current voice plan.
That's a nice chunk, especially on 2gigs, but remember the average user is somewhere less than 1gig. Even then, VZW could simply bump everyone 1gig and that pretty much covers what you were using on the calling plans. Resulting overages from calls would have, in the past, resulted in overages on minutes. But obviously it's a different story for people on an unlimited voice plan using 2500 minutes a month or something.
More than likely, they'll roll out new tiers and an allowance for VoIP is something you'll have to factor in.
For all the whining about how VZW keep sticking the existing customer, the only actual real price increase we've seen is eliminating the phone subsidy if you want to keep your existing plan.