KorvinStarmast's great answer says (emphasis mine) "The Vampire was assigned the armor class of 2, which was the equivalent in OD&D of plate mail and shield. This may have been related to its level as a monster (7-9 HD) and thus a reflection of how tough it was even for higher level characters to do significant damage to it."
I'd like to expand on this concept a bit...
That's where vampires fall in the undead power series
If you are coming to this from a 5e perspective, there is a certain amount of naturalism to AC, at least within the narrative. The narrative is 'things that are more nimble and things that are more protected are harder to hit' (and this is supported by the meta [AC is 10 plus your dex mod and can be augmented by armor]). So in 5e ghouls are AC12 because they have Dex mod+2 but no armor, and skeletons are AC13 because they have Dex mod+2 and armor scraps.
In 5e you can expect more powerful monsters to have a higher CR by definition, and the AC of the monster is one of the things that goes into calculating CR. But because there are other components to CR, and because those things vary more than more than AC does, CR doesn't really tell you much about AC. To put it simply, in 5e a tougher monster doesn't necessarily have a tougher AC, and even if it does, it won't always be in proportion to its toughness.
But it wasn't always so.
In OD&D and first edition, the 'toughness' of a monster is measured by its Hit Dice, and these serve as a benchmark for all its other abilities. In 1e specifically, the HD tell you what level of the dungeon the monster is supposed to be on and thus how powerful you need to be before facing it. Tougher monsters are, by definition, more difficult to hit, so HD is strongly correlated with AC, and there isn't really any attempt to justify this naturalistically by saying they have some better type of armor. What kind of armor you have, what it is made from and how it is made, is very important to characters, but not monsters.
Because different sorts of monsters have different builds, this basic assumption of AC being tied to HD is most evident when the monsters run in series, from less powerful to more powerful, and Gygax loved series. There is a series of humanoids, from kobold to ogre (with lizardmen being introduced just because the series 'needed' a 2HD being), there is a series of giants (from hill to storm), there is a series of dragons (from white to red), etc. And there is a series of undead.
The "natural order" of this series can be taken from the matrix for turning undead on 1eDMG 73 (undead don't make Wisdom saves to avoid being turned; rather, each undead has its own value in the matrix and a cleric of a given level needs to roll higher to turn the more powerful undead).
If we take the order of the undead in the matrix and add in columns for HD and AC, we can see that there is a really tight fit, with each increase in undead power also increasing HD and decreasing AC (remember that in first edition, lower AC means more difficult to hit).
| Undead |
HD |
AC |
| Skeleton |
1 |
7 |
| Zombie1 |
2 |
8 |
| Ghoul |
2 |
6 |
| Shadow2 |
3+3 |
7 |
| Wight |
4+3 |
5 |
| Ghast3 |
4 |
4 |
| Wraith |
5+3 |
4 |
| Mummy |
6+3 |
3 |
| Spectre |
7+3 |
2 |
| Vampire |
8+3 |
1 |
| Ghost |
10 |
0 |
| Lich |
11+ |
0 |
So, within the paradigm of monster power for OD&D and 1e, vampires are difficult to hit simply because they are powerful monsters, and being difficult to hit is a large part of what makes them powerful. As it turns out, they are not disproportionately difficult to hit – they are precisely as difficult to hit as we would expect them to be based on their order in the series – more difficult than spectres, but easier than ghosts.
Post hoc speculation on my part addressing slight disconformities in the table:
1 Zombies are easier to hit than weaker skeletons because, in one of the rare moments of naturalism in AC, they are notoriously slow. They always strike last in the round (don't roll initiative).
2 Shadows are easier to hit than weaker ghouls likely because they are the 'first' (weakest) undead for which you need a magic weapon to damage them.
3 There were no ghasts in the OD&D Monsters and Treasure booklet. They might have had to have been shoehorned in between the preexisting wights and wraiths for 1e, resulting in their odd number of HD.
