I own both, and I'm impressed with both. I recommend you keep the Glock and save for the VP9.
Here's how it will go if you sell your Glock...
1. You will find a buyer because they're easy to sell, but you'll lose $50-$75 because you're in a hurry and you have "new gun fever."
2. You will go buy the VP9 and pay a reasonable price, but you've paid tax and lost tax on the G19 that you sold.
3. You'll enjoy the VP9 very much, post a great range report here, report that you're glad you sold the G19, and shoot the snot out of it.
4. Time will pass, and you will remember your G19 as you read threads about how awesome they are.
5. You'll earn more money and go buy another G19 that's just as good as the first, but you've paid retail and tax again.
You'll end up with both either way...save some money and do it by keeping your original Glock. Ask me how I know about all of this...
New gun fever is a bitch.