I'll answer the following questions, which hopefully give you a good idea of your situation:
- If you use vtd-xml (which uses a GPL license), must your product also use gpl?
Yes. You can still sell your product, but it will automatically use gpl.
- What does it mean if your product uses gpl?
If your product is being publicly distributed (sold, offered to the public as a free download, etc) then you must make it open source. This doesn't mean you have to have a link to your product's source code available on your website, but if it is requested by someone you must be able to send them a copy of your source code
If your product isn't being publicly distributed (e.g only used by yourself or inside your own company) then you don't have to make your code open source. But obviously you can't sell your code if you don't distribute it.
Note that this is what the current court battle described in https://opensource.com/law/14/7/lawsuit-threatens-break-new-ground-gpl-and-software-licensing-issues is about. Ximpleware argues that ameriprise are distributing a product, and therefore needs to make their code open source due to the copyleft requirement of the gpl
- You want to sell your product, but not make it open source. What do you do?
You need to contact ximpleware and buy a commercial license to use their code. (their homepage has contact details for this). After that, you are free to use their code and not have to make your application open source
- You want to sell your product, not make it open source, but also not buy a commercial license from Ximpleware. What do you do?
You can theoretically make a piece of independant code, sell it, and give instructions for your users to download Ximpleware's code and incorporate it into your program. This is quite a roundabout way, and potentially a legal grey area (since I can see Ximpleware arguing that people would do this to avoid making their programs open source or paying for a commercial license, but this is probably a problem that you won't have to face unless Ximpleware finds it financially worth it to sue your company for, and even then it's a difficult case)
Overall, buying a commercial license from ximplewar would be the most "cleanest way" to go about things, but as it stands I don't think there is anything illegal with selling your software and requiring clients to download vtd-xml separately