I assume they can get from their garage into their own garden or house without crossing your land, and that the deeds say vehicle access only because there is no reason for them to use your land for any purpose other than to drive to & from their garage. Presumably, there is a clearly defined route from the access onto your land up to their garage. It would be helpful if this route had been shown on a plan with the deeds, but even if it is not the wording should be quite clear - i.e. they can come across your land only to get to & from their garage using a vehicle.
If this is the case then they have no right to walk to and fro in your garden. You have to decide how to deal with it, but flyciderman's comments are very relevant