Jay

Could be just the RNG. I would knock on the flask. I would suggest to check the vendor regularly. There was a flask that had 0.15 to 0.2 per sec recharge rate. For boss encounters where no minions/mobs spawn for recharge chance this is super useful.

For the scrolls I only use them for gold and red items. Could be just the RNG again. I typically get better rolls when using a scroll instead of the vendor.

Same goes for magic items with flasks. Check the vendor and it might have something to turn the tide in character progression. Depending on what you are looking for this is sure to come in handle.

Charms as well fall with flasks and magic items. I was humbled by the act 1 boss due to no resistance. Went to see Una and wouldn't you know it. A ice charm and two ice rings. Needless to say YES it was expensive, but the cost of those rings allowed me to progress further into the story. If you ask me, that was worth.

Lastly the belt is what I agree on here. I figured you'd have to build it up. From normal>magic>rare and hope you get some good rolls. It'll be a hefty investment.