Not sure if this pipe is acceptable. Vent pipe is a small flex pipe going into a larger pipe. There is no seal on the pipe where it is connected.
That pipe is not acceptable. Cannot be flex pipe.
It looks to me like that is a listed, flexible B-vent. I’m not sure it’s installed properly (the pic is a bit small) where it enters the other vent.
Do you have a better picture?
I agree with Jeff. Always look for a label on vent pipes. They do make flexible B-vents.
Looks like an old AO Smith water heater, hope you looked for rust around the bottom seam… Or, as I do, recommend replacement just due to the age.