A shimmering, multicolored plane of light forms a vertical opaque wall
Truesight: maybe indirectly
Monster Manual (p. 9):
A monster with truesight can, out to a specific range, see in normal and magical darkness, see invisible creatures and objects, automatically detect visual illusions and succeed on saving throws against them, and perceive the original form of a shapechanger or a creature that is transformed by magic.
Nothing in this implies that truesight allows one to see through an opaque wall. However, truesight also grants:
Furthermore, the monster can see into the Ethereal Plane within the same range.
And here is where it gets complicated. Nothing in the description of prismatic wall says or implies that any layer of the wall is made of force, and thus the wall does not exist on the Ethereal plane. However, the characters are (presumably) living, and the DMG says that "[living beings] on the [Material] hamper the movement of a creature in the Border Ethereal." The Etherealness spell states that "You can see and hear the plane you originated from, but everything there looks gray, and you can't see anything more than 60 feet away." That is, someone on the Border Ethereal can see a version of the overlapping Material plane (although the objects appear as muted gray forms) and living creatures on the Material create impassible physical barriers on the Ethereal.
Thus it seems reasonable for a DM to rule that Vecna could use his truesight to see into the Border Ethereal, where projections of the characters appear in gray tones and where the wall does not block his sight. Since Border Ethereal locations 'map one to one' with the Material, it is then possible to rule that he can use this indirect sight to target characters on the material plane, since he can, in effect, 'see their location' by noting the location of their forms that are visible on the Ethereal. It is certainly not RAW, but it is also not an unreasonable interpretation. If anyone could do this, Vecna could.
X-ray vision - no
The ring of X-ray vision says:
When you do so, you can see into and through solid matter for 1 minute. This vision has a radius of 30 feet. To you, solid objects within that radius appear transparent and don't prevent light from passing through them.
The ring specifically says that it can be used to see through "solid matter [and] solid objects"; it does not impart any ability to see through opaque substances that are not solid. It could not be used, for example, to see through an inch thick of liquid mercury, or a dense gaseous fog or smoke.
While the wall is opaque, the "shimmering plane of light" is not solid in any natural English usage of the word. Creatures passing through it may trigger damage or other effects, but it does not physically block their passage. And it is certainly not an object; it is a magical effect. Thus the ring's ability to make "solid objects...appear transparent" simply does not apply.