It's all academic until a case is bought to trial. issue is as far as i can tell manufacturing is not clearly defined and the term modifies is only used in terms of turning an IF to RIF.
So two scenarios
1) A person with out a defense buys all the parts necessary to make a function RIF and assembles it they are guilt of manufacturing a RIF.
2) A person without a defense is gifted a RIF or hasn't played for years replaces every part in the gun other than the Gearbox case, spring guide and rear sight with non like for like parts so what was once an MP5 is now a G3. yes that is modification i would say that's also manufacturing and I bet a prosecutor could make a convincing argument in that favor.
I know that's a extreme argument but even converting an M4A1 to and M16A2 means replacing over 60% of the external parts. the argument is manufacturing and modification are different things but they aren't clearly defined terms in the VRCA. Either way unless they have a nice paper trail of purchases are catching you in the act they can't prove anything illegal was done.