I may be wrong, but I can’t believe the Project Phono Box would be an audible improvement over the Vincent, which I also own. I agree, it’s not amazing but it still beats out several internals I’ve tried over the years. Overall, I think you might need to bump the budget to get an “upgrade” as opposed to a lateral move. That said, I have a couple friends who swear by the Gold Note PH-10, if you want to kick in a few extra bucks.
I can also say, I currently use the Softone Model4 (http://softone.a.la9.jp/english/Model4/m4-1.htm) which I think is a steal for $650. Sounds a lot better than the Vincent. Single tube stage. Absolutely beautiful with the LOMC. Sometimes increased cost doesn’t equate to better performance.