Lol, all of this logic is pretty flawed. Time is time regardless what you're doing with it. If you can make 15m faster than you can do an avg of about 100 VWs then make the gil and buy cells (as mentioned this can be tedious, but its not like you're super concerned about immediacy in either regard really). My ls leader 8boxes 12 pils an hr, in that case I'd probably just spam the VW. Most people take on avg 1.5hrs to build a group/kill 6 (and I honestly think that's a liberal estimate). Even if you never had to rebuild you'd still be looking at roughly 20+hrs of continuous work. If your income is at all decent, it would probably better serve to buy, regardless of whether or not your working on a relic since the ultimate time spent doing the VW would probably detract from your ability to grind gil for currency.
tl;dr roughly speaking: how much gil you make is related to how much time you put into making gil; by saying "well I'm doing a relic so I would rather VW since thats 'free'" you are basically saying "I'm really gambling on my time to hope that I will get the VW body in a short enough period of time that it would detract less from the progress of my relic than spending my time otherwise just farming gil."