It Can (and possibly should) Be but we don't have an official rule

The rules do specifically call out damage of AOE or Multi-target effects to be roll once apply to all. However, it doesn't say the same about healing AOE or Multi-target spells.

With this absence of a rule specifically about healing you can fall back on how damage works and just assume that is how it should be handled and that has one big advantage, less time rolling dice. If your table doesn't care to much for rolling dice and wants to get back to the story you just roll once. Everyone adds it to their health. Then you get back to the action.

However you can also roll for each recipient or have each recipient roll themselves. This has one big advantage, more time rolling dice. this might be preferable if your table likes rolling dice, everyone has enough dice to do it at the same time, or you are using a RNG dice rollers. This also gives the players more of a hand in how much they get healed.

The case can also be made that individual rolls for heals will be less swingy in over all healing. It can suck to do an AOE 2d6 heal and roll snake eyes, thus giving everyone in the group 2 health when the average would be 7. Where rolling for each recipient would tend closer to the average, even if one of the group only gets 2 health back.

Ultimately it comes down to how your table and GM want to handle it. It would be difficult to determine what general practice is without a poll of some sort, and baring a direct statement from a designer or an errata added there is not going to be a more authoritative than a table level.

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.