On most engines its a pretty involved job changing the oil pump. Not familiar with VX engines but Ford engines of the same vintage would require sump off and timing belt off both of which are not that easy with the engine in the car especially since the Ford engine uses the belt cover as an engine mount. Just to get to this point will require several other components removing. Many hours work.

But has the garage actually checked the oil pressure with a reliable gauge. I suspect not since they only say it "may" be the oil pump.