I also found this article about VOMS -- these are actual dealer quotes:
"GM told us it was going to be a pull system; that is, we tell them what we want. But it's not; it's definitely a push system. Another thing that's just irritating as ****: let's say you (the customer) wanted a red Suburban. You go to your local dealer to order a red Suburban. The dealer has to say, 'Sorry, but I can't order that for you.' That's a major problem with VOMS. If the dealer has earned allocation for a Suburban that week, he can change the color and option configuration of that vehicle to match the customer's needs. If he hasn't earned a Suburban, the dealer has to wait until the following week to see if he's earned a Suburban. GM allocates the dealer model numbers, body styles, and the dealer has to configure those model numbers to what you want for your inventory. But you can't just sit down and place a sold order anymore.
I had a customer who wanted a regular-cab, short-box Silverado V-8. We didn't have allocation for that type of vehicle for three weeks. We'd already told him it would probably take six to eight weeks to get his vehicle. I had to call a dealer friend of mine and ask him to let me order that truck through his allocation, and we'll just do a swap on something he needs. That's what I am having to do to get my customers the vehicles they're asking for.
The allocations are based on what the dealer has forecast in his consensus the previous month. If I tell Chevrolet I want five regular-cab Silverados, I can request them, but that doesn't mean I'm going to get them. The only way to know whether or not you'll get them is if they appear on your computer on the day of the allocation. You have to go into the computer and see what you're going to get. If the sales manager isn't alert and doesn't react immediately to the unwanted vehicles he sees on the screen, then the dealer is going to get those vehicles.
Another example: let's say it's noon Tuesday, you go to the computer to see what you ye earned for the week. If there's a chassis cab in there and you don't want it, you need to try to put a pickup bed on it because it's a whole lot more marketable as a pickup than it is as a chassis cab. If your sales manager waits a day before he tries to change that order, all the pickup beds will already be gone, and you'll end up with the chassis cab.
You can't cancel the order the best you can do is post that vehicle on a bulletin board under Pattern for Trade and hope that some dealer out there is looking for a chassis cab at that moment. If not, that baby's coming to you. The order that's generated by the factory is very generic. It's up to the dealer to go into the computer and customize each vehicle, or else you'll end up with a lot of white cars on the lot."
Reading through this, you really have to rely on the dealer to be honest with you -- and inherently, this is the overall problem with trying to order and Enclave -- too many orders, not enough dealer allocations.