Your upload speed is going to be a way bigger issue than the download speed as the lines are generally asymmetric. You can control the bandwidth Vonage uses by changing the codec and you'll get some benefits in terms of reduced bandwidth usage but you still have the potential to run into troubles if someone is streaming a video while you're making a call.

Having two different VoIP services running on the same line should not be a problem; they'll each just register to their home server and run normally.