It's a one-year deal. The Bills will pay Young a $2-million base salary, with up to $1 million available in incentives, according to ESPN's Chris Mortensen.
Young is expected to join the club Monday.
His only serious competition for the No. 2 job would appear to be Tyler Thigpen, a journeyman who barely played behind Fitzpatrick last year.
Brad Smith was the Bills' third-stringer in 2011, but his role is to run the team's wildcat package, and catch the occasional pass.
Young, the one-time star Tennessee Titans quarterback turned unwanted free agent, must have impressed Bills general manager Buddy Nix and coach Chan Gailey when he worked out for the team a week ago Wednesday.
"It's a competition for the backup quarterback job. Not the starter," Nix told reporters Friday afternoon. "Obviously I've said that 100 times. (Young) knows it. That's the way it is. It's just to try to get us better."
The Bills could not have underscored more -- and indeed have practically screamed -- the fact that Young is merely a candidate to compete to be Fitz's understudy. That's it.
For instance, asked Friday whether Young would replace Fitzpatrick should the latter struggle early in the season, Nix bristled:
"Don't even start that. Our second quarterback will play if something (injury-wise) happens to Ryan, whoever that is."
So there is no quarterback controversy in Buffalo. Period.
As I wrote last week, signing Young is risky. The last thing management wants to do is upset the special team chemistry that everyone on the team sees and appreciates.
Nix and Gailey, though, must be convinced Young will accept a backup role behind Fitzpatrick, even if he'll no doubt try to show the coaching staff in summer camp that he's the team's best QB. That's fine -- every NFLer should have that mind-set.
The crux of this whole situation, though, is how Young will behave when he doesn't unseat Fitzpatrick, whom the Bills committed to last fall with a six-year, $59-million contract extension.
Word inside the NFL is that Young has a poor work ethic -- or at least did in Tennessee. In five years with the Titans (who drafted him third overall in 2006), Young was said to be a me-first player, blind to his own culpability for his misfortunes as an NFLer.
While he eventually accepted his backup role in Philly last year, Young still said things occasionally that probably made team officials wince.
Young said all the right things Friday in three tweets he sent shortly after becoming a Bill.
"Thanks to Buddy Nix and coach Gailey for giving me the opportunity to come in and play and compete," Young tweeted.
"I am looking forward to playing with the guys and helping where I am needed. I have always respected the Bills organization and am looking forward to being a part of the team."
Between bonuses and base salary, Young earned $4 million with the Eagles in 2011. The Bills are getting him for much less.
In Young the Bills now have a QB who possesses experience, rare athleticism, a good arm and a winning NFL record as starter (31-19).
Lean and long-legged at 6-foot-5, Young was AFC rookie of the year with Tennessee in 2006, and earned a comeback player-of-the-year honour in 2009, after injuring his left knee in 2008 and never winning his starting job back from Kerry Collins.
But Young was benched late in 2010 after a heated argument with then coach Jeff Fisher. Young was done in Nashville. The Titans released him last July.
The Eagles signed Young to a one-year contract last fall to back up Michael Vick. When Vick was injured, Young started three games and looked great in one game -- a win over the New York Giants -- but awful otherwise. On the season he completed 58% of his passes for four touchdowns against nine interceptions.
Philadelphia chose not to re-sign Young. He became a free agent March 13. Before the Bills worked him out last week, he'd received barely a sniff.
Now we'll all have to wait to see if Young is up to snuff -- as a backup.
VINCE YOUNG, THE GOOD
BIG MAN ON CAMPUS: Eventually dominated games in college as few quarterbacks ever have. Tall (6-foot-5), elusive and deceptively fast, he was almost impossible to stop when he escaped the pocket and ran. One year after rushing for 192 yards in a Rose Bowl victory against Michigan, Young as a senior became dangerous as a passer, too. He capped a perfect 2005 season for Texas by defeating Pete Carroll's best USC team in the No. 1 vs. 2 Rose Bowl. Carroll threw every manner of blitzes and coverages at Young but couldn't even slow him down, let alone stop him; Young ran for 200 yards and three TDs, and passed for 267.
GOOD START: Selected third overall in 2006 NFL draft by Tennessee. Was AFC rookie of the year in 2006.
REGRESSED, THEN GOT BETTER: In Tennessee, Young's TD/interception ratio actually improved as his career progressed. Whereas it was 21/30 after his first two seasons -- which is horrible -- it was 20/10 over his last two seasons, 2009-10.
IS A WINNER: 31-19 career record as NFL starter.
QUOTE: "Because he has a lot of physical ability, every once in a while he'll make some spectacular plays." -- NFL talent analyst Greg Cosell
VINCE YOUNG, THE BAD
WHY HE HAS STRUGGLED IN THE NFL: "I think Vince Young is a quarterback who is limited in the kind of offence he can run," Cosell told me last week. "He needs to run an offence whose foundation is the run, so you give him the play-action pass game, because that limits your reads, and defines your reads."
PARTICULARLY BAD IN A SHOTGUN SPREAD: "If you put him in the shotgun," Cosell said, "he's not very good at seeing things before the snap, which is a very, very critical part of playing quarterback in today's NFL -- particularly if you're in the shotgun in a spread, because it's always up to the quarterback in the spread to figure out who potentially is blitzing."
AND THAT'S BAD BECAUSE: A spread quarterback who struggles in the "pre-snap phase" of the shotgun spread is killer because defences will overload their blitzes to one side if they know the quarterback won't make accommodations for it.
UMMM, YEAH: The Bills run a shotgun spread offence.