I'll do my best at answering.
I have a Sport, and love it. A truly marvellous thing. dd1 is SN, though, and at 5 is just coming out of a buggy, so I have had a good few years use...
afaik, the main difference between the Sport and the Vibe is that yuo can apparently fold the Vibe with the second seat still on. I think this is only when it is in rear toddler mode, but don't know as haven't ever had a go with one. It might be a bit lighter too.
Downside, imo, is the Vibe doesn't seem as roomy as the Sport. It def has a smaller footplate (massive consideration for me, as dd1 is quite old and has huuuuuge feet . there is no way she would have been comfy in a Vibe over the last couple of years)
I'd say a Sport with 14 months use is worht looking at, as long as treated reasonably well. Mine is now 2.6 years old, and has some fraying on the covers form rubbing in car boot when folded, but otherwise in ver good nick (needs a wash, mind!) no idea what a good second hand price is - maybe try looking at what they go for on ebay?
I have no accessories other than rain cover, which I have only used once. I found it a bit faffy, tbh, and go for the preferred option of waterproofing the children instead! but dd1 has issues with raincovers anyway (ASD) and so I only try to use it if caught in monsoon-type rain.
I have used a generic footmuff in the top (main) seat before, and in the second seat come to think of it. as long as the footmuff has enough slots for straps then you're ok, i reckon. it needs two at shoulder, two at waist and one at crotch. the straps on P&T's are a PITA imo, but you can get footmuffs to work ok ime.