I'm building a homebrew item based on the Eye of Vecna, but it takes both eyes instead of just one.  I'm trying to have the item remove all other senses, like blindsight and darkvision.  I've tried adding a modifier for each sense and setting the fixed value to 0 and to negative numbers, but that just changes the sight numbers.  Is there any way to have the item remove the sense altogether?  Or, since other class or racial abilities grant sight, is it impossible?

Unfortunately it is not possible. Since nothing official does such a thing, the option for it was never programmed into the system.

I figured.  I'll just keep track of any bonuses to the character's senses and make sure the negative of this item equals that.

The eyes don"T see better than normal senses?

I'm only basing it off the Eye of Venca.  In this story we're doing, a player's character will be loosing both their eyes, so loosing all their visual senses, only to be gifted this homebrew I'm making, so Truesight will be the only way they will be able to visually perceive the world around them.

Truesight is better than Darkvision and Blindsight combined, and Tremorsense, isn’t a visual thing. If they have both Darkvision and Truesight to a range of60 feet it wouldn’t make any difference.