The first step is to figure out how many people can fit in a veranda 80 ft. by 90 ft. long. A tight crowd has a person for every 5 square feet (or 5 people per D&D square). However, the entire veranda can't get filled; there's a long banquet table. With the vampire, 18 additional members of "Team Vampire," and 15 members of "Team Werewolf" invited, the table has to seat 34. A table setting needs two feet of table space, so the entire party can sit at a table that's 32 ft. long, with one person on each end and 16 per side. Let's round that up to a 35-ft.-long table to fit onto a D&D grid better and make the math easier. The table will also be five ft. wide for similar reasons, which is a little deep for a banquet table but not wholly unreasonable; it leaves plenty of space for flower arrangements. I'm sure wolfsbane will feature prominently.
So the table itself takes up 5 ft. x 35 ft., but presumably no one will be sitting on the guests' laps while mooning them, so let's leave an additional 5 ft. clear around the table. That leaves us with a 15 ft. x 45 ft. section of the veranda reserved for the guests and table. 80 x 90 - 15 x 45 is 6,525 sq. ft. With a person per 5 sq. ft., that's a potential of 1,305 people we can cram onto this veranda to moon Team Werewolf.
The easiest way to do this is with illusions. The level 3 spell major image can create a force of people filling out an area of four 10-ft. cubes plus one cube per caster level. At CL 10, that's fourteen such cubes. Assuming no one will be standing on each other's shoulders, we can disregard the vertical element and just say it's fourteen 10-ft. x 10-ft. squares per cast. We'd need 72 such squares to cover the veranda, less 8 or 9 because of the table area we're leaving clear. That means we need 5 castings (well, 4½) of major image at CL 10 to fill the veranda with an illusory crowd.
A CL 10 scroll of major image costs 750 gp, or takes an evening of the vampire wizard's time to create. He can create four such scrolls and then cast the fifth major image himself, or simply craft five scrolls. Each casting of major image requires concentration to move, so five separate spellcasters are required (if the vampire were level 11, he'd have access to programmed image and could do it all himself). If the vampire's minions are low enough level that they'll have trouble passing the caster level check to read a CL 10 scroll, a safer but more expensive option is to craft or buy a wand of major image and then pass it around. Exactly how much that costs will depend on the CL of the wand, and how high that needs to be will depend on the number of minions the vampire has who can activate an arcane wand.
More generally, a single casting of major image can create a force numbering (CL + 4) x 20 medium creatures. They won't be able to leave their squares, but can shuffle around and gesticulate and so forth.
A necromancer can control CL x 4 HD worth of undead using animate dead. There are plenty of ways to optimize this, but they involve taking feats or prestige classes, not the sort of thing one does within two weeks and for the sake of a prank. Additionally, skeletons probably lack the anatomy to moon people, so the 2 HD zombie will be the choice of minion here. That means our vampire wizard can only control 20 zombies. Even if all of Team Vampire were also level 10 spellcasters, that's only 380 zombies, which is well short of filling the veranda.
There are other ways to get an army of undead. One could try to convince a friendly Mohrg to lend you a hand, or a Pale Master or Yathrinshree; all of them can turn anything they kill into zombies under their control without limit, so if they aren't already rolling around the countryside with a zombie horde, our vampire would just need to spoon feed them 1,300 peasants. This, however, seems like it's not appreciably different than just hiring people; it's not really a display of the vampire's skill or power.
Ultimately, wizarding necromancy is more about energy drain and uttercold than it is raising minions; that's more clerical necromancy's purview.
A single vampire can control up to twice its HD in children and spawn. To be an actual vampire one has to be at least 5 HD, so within 1d4 days our level 10 vampire can control four other vampires. Each of them can, in turn, control two vampires. Every 1d4 days (the time it takes a new vampire to rise), the network of vampires owing allegiance to the level 10 vampire doubles.
Let's pretend the d4 is always statistically average; it always rolls a 2½.
- Day 0: Create 4 level five vampires.
- Day 2½: Those 4 vampires awaken and promptly create 8 more vampires (for a total of 12).
- Day 5: Those 8 vampires create 16 more (for a total of 28).
- Day 7½: Those 16 create 32 more (for a total of 60).
- Day 10: The 32 vampires probably don't have time for another wave of spawn, but a few of the 64 new vampires will awaken in time for one of the weirdest introductions to their new bloodline.
In 12 days, the vampire can create 80 or so new vampires (all level 5, the minimum for a vampire). Presuming all 80 of those vampires were clerics, they could then animate dead enough zombies to fill the veranda.
Alternatively, given another week, the vampire's network of children, each controlling two other children, would exceed 1,000. In 23 days, it would on average exceed 2,000. Vampires are very much an existential threat in D&D, especially since they don't go hungry or anything so there's not much preventing them from just turning everyone into vampires.
This option doesn't really work. Summon monster spells don't summon more than 1d4 + 1 monsters at a time, and the long-term conjuration options are planar binding—which isn't much different than domination—or planar ally, which is a cleric spell. Conjuration in 3.5 is about having really high quality minions, not masses of mooks.
The Best Option
The best option here is probably just illusions. It's easy, it shows off the vampire's arcane ability (and, if the minions make an effort to hide that they're also casting the spell, makes him seem like he's much higher CL than he actually is), and doesn't require massacring the local peasantry.
If, however, you want to escalate the severity of the vampire threat, having the new vampire on the block promptly set about creating an army of spawn could certainly do that. Rumors of disappearances, maybe even some familiar faces among the people mooning Team Werewolf—it could be interesting, but would, I suspect, mark a definite shift in tone.