How do I connect a Class B vent to this heater
A common way is with a single wall oval to round elbow. Then you use a listed adapter for joining single-wall vent to Type B round vent. Do not "squeeze" or "bend" a pipe, fitting, or adapter to make it fit.

Contrary to some things you might hear, gas fired appliance vent pipes and fittings must be fit for purpose and used according to code and manufacturers' instructions or you could have a fire or die from CO poisoning. There are all kinds of restrictions; length and slope of horizontal runs, number and degree of bends, requirements by space/area, passing through floors/walls/roof, height of vent stack, type of cap, etc. All the rules are there for a reason (usually the reason is that someone was injured or killed).