Any fax machine will do. The way a fax machine works is basically the same way your phone works, it dials a number using tones, then when connected (somebody answers) it starts communicating ("talking") to the fax machine on the other end by sending a series of tones, these tones tell the other fax machine "hey hows it going, I have to give you something, ready? okay start writing."
The only thing you may need to be concerned about is if you have the right connection to send faxes, some ip phones will "back-feed" the other phone outlets in the house and therefore when sending a fax it'll need to be hooked up directly to the modem to send and your other phones will be disabled while doing that, others will merely be an "access" point where all the phone-lines run through it and then you'd just need to plug in the fax and away you go.
Check with Vonage to see if you need to do anything "special" to the modem to allow data transfer from a fax to go through, usually you don't.