What type of vacuum hose is it? Shop vac? Is it really a 2-1/2" hose or is that just the fitting that connects to the vacuum? I use 1-7/8" hoses with ends that fit my 2-1/2" shop vac inlets. The part that fits into shopvac accessories fits into a 2" Fernco and a 1-1/2" PVC pipe should fit into the end of a 1-1/2" Fernco fitting.

So, I use a 2" to 1-1/2" Fernco coupler to attach my 2-1/2" shop vac inlet via a 1-7/8" hose to a 1-1/2" PVC pipe.

Making a cyclone dust collector?