Welcome to the forum!
Congratulations on starting at such a young age and for reading a good investing book. As far as your questions…..
1. I think comparing Vanguard to Fidelity is like comparing apples to apples. Not much of a difference. Both are good funds with low expense ratios. I personally use Vanguard for no other reason then that was the one I stumbled upon first that had a good index fund selection with low expense ratios.
2. Once again both companies offer a nice selection. Vanguard has the Target Date Retirement funds which have a minimum initial investment of only a $1000. Fidelity also has target date funds although I'm not as sure about the expense ratios. Within each target date fund from Vanguard is US stocks, International Stocks, and Bond funds all with certain percentages based on your desired asset allocation.
3. You definitely want to be concerned with your asset allocation. At such a young age depending on your risk tolerance, i.e ability to lose money when the markets are down, I think you can have a greater allocation of stocks than bonds. Maybe 80/20 or 90/10 if you have a greater risk tolerance.
You've got some thinking to do. Read the wiki here and take a look at the 3 fund portfolio. Keep asking questions