Glad you have good boots - Vasque is a great brand. My first real boots were Vasque and I wore them for 15 years. I disagree with your friend about the softer soles shoes when you are actually on Half Dome. I prefer the overall support of a hiking boot for any hiking in the mountains, especially over rough terain. The stiff sole makes stepping in uneven rocks more confortable and the ankle support is important also. But it is a personal preference thing.
You will probably be camping in Little Yosemite Valley as your starting point for the day-trip up Half Dome. I'm not positive about the mileage, but I remember that portion of the trip as being the steepest part (OK it was over 20 years ago and I was a teenager at the time, but mountains don't change much over the years). There is a large elevation gain from there to get to the top of Half Dome. The actual last 1/2 mile to the top is a small part of the hike - where the infamous cables are - and I think you will appreciate the sutrdier boots for the rest of the day.
Either way, you will want to bring another pair of footwear anyway. You poor feet will want different footwear to put on while you are not hiking and in camp. And if something happens to your hiking boots you need an alternate to wear.
As for the safety issue of leaving your big pack unattended while you go up Half Dome, that is your call. I agree you do not want to attempt the dome part with a full pack. The area you will leave your pack is one of the most crowded hiking areas of the park and that makes it just like a little city, with all the crime that comes with lots of people in one area. Check with the park rangers and get their opinion. At minimum, get one of those locking net things that will let you lock your pack to good size tree.
As for advice about the pack, I cannot be much help. While I have done miles of backcountry hiking, in the last 15 years I have tended to stick to long day hikes and am not up in the latest in large packs.
But I think fit is probably the most important factor. If the larget pack has enough cinch straps, you can always cinch it down when it is not full. A pack that doesn't fit right will make your trip much less enjoyable.
You said the salesman you talked to was not very helpful. Find another salesman. You must have gotten the new kid or someone who does not backpack. REI usually has very knowledgeable staff. Try going at a time when the store is not very busy. Or go to another store. Sorry I don't know the stores in California area. Just because you may be buying the pack from where your friend can get it, doesn't mean you cannot to to another store for advice.
Or go to the library and research back issues of Outside, Backpacker, etc magazines. They usually to product reviews every year and even if you do not end up buying the exact model they review, their reviews usually give good information as to what features to look for. Or I bet there are message boards out there for backpackers. Start with the magazine web sites or the Sierra Culb.
Be sure to check back in when you return and tell me all about your adventure !
backpacking?
