This is not infinite HP, this isn’t even decent healing.

You’re never going to heal for more than half of your Arcane Ward’s HP (or your Wizard level + 2, assuming you have a +5 INT modifier). Keep in mind, at-will casting is not a “free action”. You’ll need to burn an action, or 6 seconds per casting of Mage Armor. In other words, it takes a minute to restore 20 HP to your Arcane Ward. Therefore, Vampiric Touch will run out duration before you can replenish your ward.

Using a third level spell slot, you may regain up to (assuming you have a perfect damage roll):

10 Hp at level 8 12 Hp at level 10 14 Hp at level 12 16 Hp at level 14 18 Hp at level 16 20 Hp at level 18 22 Hp at level 20

You won’t actually heal that much though, because it’s highly unlikely you’ll roll damage exactly equal to your Ward’s HP. You’re most likely going to either A: deal extra damage to yourself, or B: roll just under your Ward’s HP, and come up a few points short.

For perspective: On average, Cure Wounds cast with a 2nd level slot restores 14.5 HP. 18.5 HP with a 3rd level slot.