You might look up the investment order post/thread here and see if it suggests something different for your situation.
In general I think it's a reasonable plan. But since you asked for cons, here are the ones I can think of:
1. You'll have an extra account to keep track of, with all the attendant hassle of opening it, funding it, dealing with monthly statements, etc.
2. You'll have to keep track of and pay taxes on any dividends, capital gains distributions.
3. If VTSAX is low or down in 9 years when you want or need the money, that's something to consider. Some might suggest a more balanced fund, but that's your call.
4. If you end up using the taxable for 529-eligible expenses (presumably because the 529 balances are already used up and you still have 529-eligible expenses), then you'll have lost some tax efficiency. From your post it sounds like you're OK with this con.
5. The investment order might suggest something else as a higher priority. The only thing I can think of offhand is an HSA (which I'm not even sure I agree with 100% but I understand the recommendation), but there could be others.
Nothing wrong with making multiple withdrawals from a brokerage account. You hopefully know that in order to make a withdrawal, you'd probably have to sell some of the VTSAX (or whatever), and selling shares of VTSAX (or whatever) would result in a capital gain or loss being realized, which would be a taxable event and you might owe capital gains taxes at that point.
