@Apples521 

"Is it incorrect to just reduce the expense account by the discount amount? Ultimately, I'd like to handle this at the point I enter the invoice as creating a credit memo is an extra step and nuisance."

That's totally acceptable.  You definitely don't need to enter a vendor credit.  How you record the discount is up to you in terms of how you want to see it on your P&L and reports.  Personally, I just discount the amount by the % discount when entering the bill.  For example, if your bill is $1234.56 and you get a 2% discount, just enter 1234.56*.98 and let QB do the math for you.  If you want to see purchase discounts as a separate line item on your P&L, then you will need to enter the discount as a separate line item on the bill or use a credit memo.