I haven't noticed any particular seasonal sales on vinegar, but at the bulk store where we get most of our kitchen stuff a box - three gallon jugs - is around $3 - $4 any time. (Quite a bit cheaper than bottled water.) I've used it with some success on weeds that sprout through the cracks in the driveway, but enough of it to be effective acidifies the soil enough that nothing grows there for a rather long time. It might be useful for "edging" but I'm not sure I'd recommend it for clearing a new garden space to get ready for planting.

Effectiveness of any treatment varies a lot with whether the plants are in active growth, new growth, mid-season, or going into dormancy, and some work at one time but not at others. For materials intended for specific uses, reading the recommendations on the package carefully is recommended. Sadly, my bulk vinegar doesn't even come with decent pickle recipes on the label.

John