The 28-year-old Belgian is a talented defender with the potential to be one of the best in Europe, but the dominance of Per Mertesacker and Laurent Koscielny last season left Vermaelen firmly on Arsene Wenger's bench.
Enter Daniel Agger.
The Liverpool man fell out of favor under Brendan Rodgers last season, and the arrival of Southampton centre-back Dejan Lovren has left the vice-captain desperate for a transfer, according to Darren Lewis of the Mirror. Barcelona is keen to sign the 29-year-old, but Arsenal would do well to beat them to it.
As good as Mertesacker and Koscielny are, they can't be expected to shoulder the load for an entire season, and there are no serious in-house candidates to fill in for them.
New Arsenal singing Calum Chambers has been deployed as a centre-back in the preseason but it is not his natural position, not to mention he's only 19 years old. Besides, David Wright of the Express is reporting that Wenger may transform him into a much-needed defensive midfielder to presumably back up Mathieu Debuchy at right-back, so he would only be used as a centre-back in an emergency.
Agger, on the other hand, brings a great deal of talent and experience. He has made over 200 appearances for Liverpool and has been a staple of Denmark's national team. He was voted Denmark's player of the year in 2007 and 2012 and earned a spot in Brondby's starting squad at just 19 years of age.
Agger reads the game well, has composure in the air and is a threat to score when he moves the ball forward. At Liverpool, his leadership earned him a vice-captain role, and he may have been a longtime captain if Steven Gerrard didn't have a lock on the designation.
There's no question he would be a quality addition to any side, and Arsenal's back line would be solidified with Agger supporting Mertesacker and Koscielny.
But there's a catch: Agger can't seem to stay on the field.
He missed most of the 2007/08 season with an injury only to have a back injury sideline him for two months soon after reclaiming his starting spot. In 2010/11, a concussion and other nagging injuries kept Agger on the bench for the first half of the campaign. Other seasons have seen him on and off the field, his week-to-week status often in doubt.
In all, he's only started about half his potential games since debuting for Liverpool in 2006, which has derailed an otherwise promising career. As Sports Illustrated's Jonathan Wilson put it in 2012: "If it weren't for injuries, Agger might already be hailed as one of the greatest central defenders in Premier League history."
This spotted history shouldn't dissuade Wenger from signing him. Liverpool lost patience with Agger because he was expected to hold down the centre-back position long-term. Injuries kept the Reds scrambling for replacements, and capable substitutes like Martin Skrtel made Agger nonessential. That won't be a problem for Arsenal.
Since Agger will coming off the bench, he should be shielded somewhat from sustaining injuries that build throughout a long season. Wenger will carefully monitor the Dane's fitness and would not have to employ him when a weary campaign leaves him open to injury. And if he is injured, Arsenal loses a capable backup, not an essential cog.
At best, Arsenal can rotate three centre-backs without losing a step in quality. At worst, Mertesacker and Koscielny can hold down the fort as Wenger finds a new substitute. There's too much upside to ignore.
Wenger will hope the injury history scares off other suitors. If Liverpool is asking too much for Agger and Barcelona doesn't make a serious bid, Wenger can rest on his laurels and wait for a price cut. Agger would be a tremendous boost, but he's not a pressing need.
Arsenal needs to make a move to replace Vermaelen. Agger's intelligence, technical ability and leadership are simply too valuable to ignore.