Copying my complete reply here, because the Mod moved it while I was still editing it:
They can't tell whether a car is VLI or VLII, but they can switch you to a specific car number if space is available. Make sure they are doing a change, not a rebooking, and that the cost will be the same. If they tell you the cost will be higher, ask to speak to a supervisor, who should know how to do it without a cost increase.
BTW, this may have already been said in this thread, but to reinforce, agents at the AGR number tend to be more experienced and knowledgeable than agents on the regular 800-USA-RAIL number. The wait time is usually shorter, too. If you haven't already, you can and should sign up for AGR immediately; it can be done onlne and costs nothing.
The sleepers so far have been:
10 | VLII |
11 | VLI |
12 | VLII |
13 | VLI |
14 | VLII |