I don't think this is something you can control. From what I understand, messages from Yahoo or MSN are treated as txt messages by Verizon. Nothing you can do about it.
Ok, I have seen several posts about this today and I'm dumbfounded why you think these services are being charged to you as texts? I've been using Yahoo IM, Nimbuzz, AIM, etc for 8 months and not a single MMS/SMS charge as a result.