1. Do I have to use the Accounts Payable module?
No. But if you do it will give you a more complete accounting system.
It will facilitate paying the bills, keep track of money owed, and allow the system to automatically handle Cost of Goods Sold.
2. How do I make sure my vendor numbers are different than the account numbers I use for my customers?
It's a good idea to use different ranges of numbers for each.
For instance, you might have your customer account numbers begin with 10000 and vendor numbers begin with 2000.
This is easily done by entering different starting numbers for each in Company Preferences.
See Set Up Company Preferences (Accounts Receivable) for help on this.