Have you actually played 5.5, by chance? Your math makes a lot of generous assumptions that have zero chance of holding up in actual play, in my experience. Monsters had absurd offensive buffs.
A melee martial gets stomped so hard in actual melee in non-trivial encounters that you'll be lucky to not be rolling death saves on turns 3 and onward. That rules out pretty much any usage of Hex and Hunter's Mark.
Also, to use the guidance from 2024's DMG:
- A Rare item should cost 4000 gold (plus cost of the base item)
- Common magic items can often be bought in a town or city. Uncommon and Rare magic items are usually found only in cities (...). If you decide to allow purchasing items, you can limit them to cities, but you can reasonably buy a Vicious weapon in a weapon shop.
- You're supposed to roll on a Treasure Hoard about once per session; Roll twice or roll once or double the total for monsters fond of amassing treasure. You use the highest CR to determine the treasure range. A CR 5-10 has a hoard of 8d10 x 100 (4400) gp and 1d3 magic items. A CR 11-16 has a hoard of 8d8 x 1000 (36000) gp and 1d4 magic items.
Combining that, you can easily see your party should earn over 4000 gp per session starting at about level 3, and over 36000 gp per session starting at level ~7 (at least, that's the earliest level you can reliably destroy CR 11 bosses and their mooks).
So, you will have strong weapons pretty early, you will get upgrades often, and considering that martial class progression is still absolute ass after level 5-6, this is completely fine and by design.
Besides, there's plenty of items that are vastly stronger than Vicious weapons - Enspelled items, wands, consumables, +AC shields and armor, resistance rings, most of which aren't attunement in 2024 either. Vicious weapons are strong, yes, but their function is mostly to be an excellent sidearm to use your weapon masteries with, and to ensure your secondary mode of damage (ie. Vicious Longbow for GWM Barbarians) is not absolutely garbage against the HP inflated 2025 Monster Manual.
The only change I'd make to Vicious weapons is to introduce greater variants with a Very Rare +3d6 damage, and Legendary +4d6 damage, also without attunement requirements.
TL;DR: Vicious is OK, there are vastly stronger items, would add a +3d6 Very Rare and +4d6 Legendary variants without attunement.