Usually DSL modems are configured by the ISPs to have an unbalanced ratio so that downloads will be much faster than uploads. The reason for this is that people doing things like web browsing (which is what most users appear to do most of the time with their internet connections), and sending a request to a web server requires very little data compared to what often gets downloaded (when viewing videos, and listening to music or radio stations, this becomes quite pronounced because one simple request to a web server starts a steady and prolonged stream of data downloading).
So, where a ratio might be 10:1, a VPN would likely be effected because data transfer needs may be bi-directional (e.g., load a document, edit it, then save it) in nature unlike most web applications [that don't involve uploading].
You'll need to contact your ISP to find out what your specific ratio is because it may vary between ISPs. Chances are that the first-level support probably has no idea how to answer this question either, so you will probably have to get your question escalated to a higher level of technical support. (The "Network Operations Centre" might be who ends up answering your question.)
Some ISPs may provide an upgrade option (usually with a higher cost) to provide higher speed access. The cable providers usually do, but for DSL it's difficult to say because there are some known hard limits, and so you might have to look at other options that are not DSL based if you're already maxing out the technology.