Would you have to sterilize/pasteurize somehow after soaking garlic, dill, or whatnot in pg/vg?
(The following assumes PG is better for extraction than VG.)
Dried dill wouldn't worry me too much. Granulated garlic or properly dehydrated garlic wouldn't either.
Real garlic has too much moisture to be safe. And the pH isn't low enough. Here's the deal:
Blostridium botulinum (botulism) is one of the types of bacteria that love the lack of oxygen. Which is why you should never put real garlic in olive oil for salad dressings. This is also why you need to be so careful when you are canning foods or using reduced-oxygen packaging. Yes, oil and PG are different. But wet garlic is still wet garlic.
Garlic has a pH of about 5.8. You have to go down to 4.6 (more acidic) or lower before the pH barrier kicks in.
Moisture content is measured as "water activity." You have to get the water activity value to .85 or less so bacteria don't grow. Jams are around .75 to .80. The "wettest" honey is around .75. Dried milk is .20. Crackers are around .10. Even people that don't dehydrate their own fruit well enough can find their product getting moldy... and I've seen it. Properly dried fruit is around .55 to .80.
I have steeped garlic (the real steeping, not the let-your-juice-sit-in-the-cupboard steeping) in Everclear and strained it out, but ethanol is an entirely different animal than water. I say this, but I am absolutely NOT advocating ethanol for anything related to vaping.
To fully prove my concern about all this, I'd have to give you more stats about PG. But there's no need, when there's granulated garlic on the market.
One final bit of paranoia: One of the chemicals responsible for garlic's aroma/flavor/pungency is allicin. How does that react to PG or VG/PG, and does the combination turn it into a different chemical? Because we'd be inhaling the resulting juice, I wouldn't try using any form of garlic until I found the answer. This is also why I won't try any form of real-food flavor extraction until I learn more.