It's a scam just clean what you see and check the EPAs site on it.

Repainting our house and took the vent covers off. Return air vents are stuffed with big clumps of dust and dirt. Probably have never been cleaned. Do we need to hire a duct cleaning company and if so, could you recommend one that's not a scam, or is it sufficient if we just vacuum up the dust near the vents?