I have not placed an order through Vons delivery, but I have through Safeway, which is the same company (just called Vons in SoCal.) Three to five days ahead of time should be fine, especially if you want to lock in your ideal delivery time. If you wait until closer to the time, you may not be able to get the exact date your want to. You can schedule out the delivery a few weeks in advance, but I don't see why that would be necessary. Just looking at my Safeway's online delivery schedule for today, tomorrow's schedule is all booked up and so is some of the times the day after that, but everything three days and over out is available.
I have always found the delivery people to be helpful, courteous and prompt. Just make sure to check your receipt against what was delivered. Once or twice I was charged for something they forgot to pack. They were always very apologetic and reversed the charge.