There are more considerations as well. Understand that a move...any move...will likely cause grief for your customers if not done right with notifications and proper planning ahead.
The best bet is talk to your customers...make sure they understand what's going on. In my book..going from shared to a vps is no "upgrade" at all..simply because it may not be in many cases.
Your customers may find themselves with fewer resources than that had before...and they will leave..it's just as simply as that.
In my book...the only upgrade is straight to a dedicated server. I fully understand you may not be ready for that..but sometimes life throws this at us..and life does not care if we are ready or not...
The biggest mistake is making a move...any move... without talking to your customers first. Evaluating and comparing different hosts for whatever it may be..dedicated servers or vps's..depends on a great many different factors.
Do you have a mix of international cusomers? Or all your customers based domestically..in the same country as your current shared server account? Take a look at what you have right now...where the server is..and what kind of network your current host has...and then go from there.
it just takes time, practice and research to learn you may be right for you. There is no "perfect" provider. If downtime is a concern..you may want to go with someone who has a track record of little if any downtime.
Make sure you understand what SLA's are in place and how you and your customers will be compensated in such a event. Understand the contracts as well...as many require a certain period of time to cancel different services.
Experience is the best teacher..and you'll understand that later. It just takes time and patience. A little bit of luck does not hurt either.
>>>Some hosts can migrate ip addresses. This is a very good thing if you are growing fast. Ask about this..it's certainly a very good thing.