When somebody talks about vendor lock-in, in general they're talking about the user's data becoming unavailable to them other than through the vendor. We have at all times made sure that all your data is available to you, either because it is on your computers directly, or, in the cases where you've explicitly chosen not to have it on a subset of your computers, your data is accessible via published Free and open standards and protocols; in most cases (in all cases except one) these standards are not even our making.
There is another issue with some cloud providers that you might think of as vendor lock-in where the protocol might be Free and open and published and all, but the client is closed and the standards and protocols governed in such a way that in effect makes it so impractical it's impossible to retrieve your data. Again, with us you always have your data unless you've elected to not synchronize to a particular device; complicating your case even further, the clients are all DFSG-free software. If you go to the Ubuntu One project on Launchpad, you can find all our clients; our service also relies on Free, open clients from other people, which I can point you at if you want.
Some of our server-side bits are closed (most of them are Free, open, published, etc). You are free to not use the service and, if you are currently using the service and want to leave, you have access to all your data at any time through a variety of means. We'll be sorry to see you go, but it's your decision.
Thank you for reading.