The breathing restriction on the vn750 is not the stock exhaust, but rather the air intake. Only when the intake is modified with an ear shave and putting on freer flowing pod filters is rejetting required.You might not HAVE to re-jet, but today's bikes are already very lean from the factory, and adding a less restrictive exhaust just leans it out more. It's a PITA to re-jet a Vulcan, but there are significant gains to be made by doing so when swapping out the exhaust.
You should be able to search jetting, and find out what people are doing.
Good Luck!