My institution has a fellowship so I can speak to it very generally. Any specifics and you'll need to talk to people involved in programs.
Pros:
-A respectable alternative to hospitalist work as a post-residency position if do not feel you're competitive enough for Cards fellowship as you'll be doing clinical work while hopefully working on some research to boost your application.
-I can not remember what it is off the top of my head but there are some board exams this fellowship allows you to sit for and there are certain tests (billable) you are trained to interpret and there may be some very minor procedures you can be trained to do. If you feel like you are going to utilize that training in general practice then it may not be a bad idea, but I don't know about the business side to that.
-At super-specialized academic centers, there is a niche for vascular medicine so if you're really niche is academia, this may not be a bad option.
-The fellowship you mention is 3 yrs, but the one I know of is 1 year and for the amount it covers, it's not a bad deal.
Cons:
-Non-ACGME fellowship with no established specialty afterwards.
-It won't automatically qualify you for higher compensation than a hospitalist or open up new opportunities to you outside specialized places.
-Many other fields are traditionally trained in areas you're trained in. Vascular Surgeons know how to medically manage PAD, Cardiologists know about venous stasis as a consequence of HF as well as AF anticoagulation guidelines, Hematologists learn HIT on the inpt side, PCPs alone are qualified to follow a patient's coumadin clinic, and the list goes on as all specialties already are trained to manage the vascular complication of their organ system.
-Expanding on the above point, but with most other fields taking the vascular complications of their fields, your bread and butter will be post-hospital discharge visits for DVT/PE the anticoagulation plan hospitalist will inevitably refer their DC'ed patients to. This management is not really super challenging and you're not really establishing a long term relationship with or engaging in significant medical complexity. Furthermore, warfarin creates a lot of that complexity and while we still need to know everything about it, I suspect with more research into DOACs, traditional indications for Warfarin (ESRD, APLS, >40 BMI) will start to disappear.