Without seeing your installation, it's hard to say what your
best option is. If the ducting is only off by one half inch,
(or less), you can use a high heat self fusing silicone tape
(Cole Parmer makes a good self bonding tape for this sort
of problem). The tape is 1" wide by about 30 feet in a roll.
The tape self fuses in 24 hours and is inseparable to the
surface it's bonded too. There usually isn't a lot of intense
heat in stovetop vents, but I'd still recommend high heat
silicone tape for connecting ductwork. It's good for up
to 500 degress F. I hope this helps.