The invocation doesn't change how vampiric touch works.

The description of the warlock's Gift of the Ever-Living Ones eldritch invocation says (XGtE, p. 57; emphasis mine):

Whenever you regain hit points while your familiar is within 100 feet of you, treat any dice rolled to determine the hit points you regain as having rolled their maximum value for you.

Note that this only applies to dice whose purpose is to determine the HP you regain.

The vampiric touch spell description states, in part (PHB, p. 285; emphasis mine):

Make a melee spell attack against a creature within your reach. On a hit, the target takes 3d6 necrotic damage, and you regain hit points equal to half the amount of necrotic damage dealt.

Here, the dice rolled simply determine the damage you do to the target; the resulting healing is simply derived from that value. Thus, Gift of the Ever-Living Ones doesn't interact with it.

Rules designer Jeremy Crawford supports this ruling in his answer to a similar question about the invocation's interaction with the enervation spell (XGtE, p. 155) - which similarly deals necrotic damage and heals half the damage dealt - in an unofficial February 2018 tweet:

Gift of the Ever-Living Ones (lock invocation) states: treat any dice rolled to determine the hit points you regain as having rolled their maximum value. Would this effect make an Enervation cast max roll every time because of it's healing effects?

If you cast the enervation spell, you regain a number of hit points equal to half the amount of necrotic damage the target takes. In other words, you're not being healed directly by a die roll; you check to see how much damage the target took, halve that amount, then heal.

The vampiric touch spell's "healing" works the same way: you make a damage roll, and then halve that amount and heal. As such, vampiric touch's "healing" would be unaffected by the warlock's Gift of the Ever-Living Ones eldritch invocation, as the actual roll being made is for damage, not healing - you just regain HP worth half of the amount of the damage dealt to the creature by vampiric touch.

The intent is that you can only damage one cursed target (and creatures next to it) at a time

The invocation repeatedly refers to a single cursed target

As you note, the Maddening Hex eldritch invocation says (XGtE, p. 57; emphasis mine):

As a bonus action, you cause a psychic disturbance around the target cursed by your hex spell or by a warlock feature of yours [...] When you do so, you deal psychic damage to the cursed target and each creature of your choice that you can see within 5 feet of it. [...] To use this invocation, you must be able to see the cursed target, and it must be within 30 feet of you.

The feature doesn't explicitly say the damage is dealt to only one cursed target, but it repeatedly refers to the target in the singular (i.e. "the [cursed] target", or "it"). This suggests that even if you do have multiple curses active - such as a hex on one creature, and a Hexblade's Curse on another - you simply choose one of these cursed targets when you activate Maddening Hex, dealing the damage to only that creature and other creatures (of your choice) adjacent to it.

Designer intent indicates that Maddening Hex triggers off just one of the creatures you have cursed

Rules designer Jeremy Crawford answered this exact question in an unofficial tweet from November 2017:

For Maddening Hex, does the psychic damage trigger off of every target cursed by you within range, (One creature cursed by Hexblade's Curse and another cursed by the spell Hex, or the like) or only a single cursed target per turn?

Maddening Hex works on one cursed target at a time.

Crawford's unofficial ruling seems to match the wording of the feature. You may have multiple targets cursed at a time using different warlock features and/or the hex spell, but Maddening Hex only activates on one such target. As you point out, it says "the [cursed] target", singular - so even if you've cursed multiple targets, you can only use Maddening Hex to deal damage to one cursed target (and creatures adjacent to that one, of your choice) at a time.