You must read TOS before buying a vps. This is one of the most common newbie mistakes which happen all the time. Don’t believe what you seen in their offers or web site. So many people misunderstand what “Unlimited” means in hosting business and many other things. If you are going to buy a vps server then you must read the terms of service and fair use policy if there is one. You can find both in their website and if you can’t find it then content them and ask them about it.
Also in TOS if you find anything which is not very clear to you then contact them by email and ask about it. You might if great vps offers and many low cost vps providers offering really good servers but sometimes what you get is not what you think you are getting. You have to ask them and find out everything before paying for you vps. Remember you are paying for it so it’s your right to find what exactly you get for your money. Don’t be shy about it.
Another thing is rather than asking in their online chat use e-mail for asking questions. That way you will have a proof to show if something goes wrong. Their ticket systems are find if you receive notifications with reply.
I have seen my try to use offers which had money back guarantee. Not every VPS provider offer money back guarantee and some offer this only if they see it’s fair. Make sure what they have it in their TOS about it. I have seen so many people rank and complain about this in popular web hosting forums to find providers TOS says otherwise.
Not all VPS providers allow you to host adult materiel. For example HudsonValleyHost does not allow any kind of adult materiel in their VPS servers even if it’s something legal. They only allow it in shared hosting packages. So if you have Adult sites then contact them and find out if they are fine with those sites you have. It’s better than asking for refund later.
Also find out what they say about backups. Some do take free backups but that’s only for their internal use like in situations related to hard driver failures etc. You always has to take your own backups no matter what they say. Don’t risk your site content because they mention about backups in their TOS.