I don't know if the vorpal sword will be a problem. I've never seen one in play.
I do know - from experience - that the wand of lightning bolts was a problem for me. The OP says it was a problem for him, too, and another player at his table. It looks like you keep ignoring that we're talking about our actual play experience.
You say it's feature, not a bug. And in a way, you're right. The thing is, I found the feature didn't work as intended.
What I am saying is it IS intended.
It isn't like it is a subtle thing, right? Starting a dungeon you're adding 6 or more fireballs to the arsenal. When a PC only has a few 3rd level spells, that is an obviously big thing.
And there are articles from the time of release of the edition talking about how fireball and lightning bolts are INTENTIONALLY amongst the strongest 3rd level spells.
They knew EXACTLY what they were doing.
They knew that if you get your hands on a wand of fireballs below level 9, it is (likely) going to be a big thing. You jump to the word 'problem' to describe what you see. What you see is a spellcaster throwing fireballs at enemies over and over in a rain of destruction. You see them rising to a starring role in combat damage at levels 5 to 8 as their fireballs are outclassing the damage contributions of the weapon wielders.
This is not a problem. This is a heroic part of the groups story. This is that character's time to shine. 10 years from now the player will fondly remember how he blades through the Hidden Shrine with his Wand of Fireballs and leveled the enemy right and left.
What about the other players? They're marginalized if one PC is shining!
Darn tooting.
Perfect balance between all PCs is BORING. That was the main criticism of 4E, right? That everything was so balanced that it all melted together and nothing felt distinct? 5E allows PCs to have a time to shine. The ranger with the Wand of Fireballs may be shining at levels 5 to 8, but the Warlock with the Staff of Power is going to outclass them at higher levels. Some PCs may have more times to shine, but I have yet to play in a 5E game where there was a long lived PC that really felt like they never did anything useful/fun.
As I understand it: You do not like how the feature works. You don't wand the wand wielder to be stronger than other PCs at levels 5 to 8. You want more parity between the PCs. (If not exactly how you feel, that seems to be in the ballpark). Fine.
However, that is not a failing of the item or system - that is a preference that works against the design of the system. They intended to have PCs (potentially) find these items early on (they put them on the tables they designed) - and they knew how powerful these items are (it is obvious) - and you can have a
wonderful experience running the game exactly as designed with a wand of fireballs in the hands of a ranger, warlock, eldritch knight or eladrin.
D&D is an RPG. A role playing game. Characters play a role in a story. Finding a powerful and iconic item is a keystone of many great stories. You can choose to have a Wand of Fireballs in the hands of a 5th level party be a great story opportunity. Or, you can elect to diminish what makes it such an outstanding thing and try to balance the story so that it has less of an impact and everything is more balanced and neutral.