just saying houndoom is a very hard counter to victini with immunity to both stabs
It's more of a check than a counter, really. A counter is commonly defined as a Pokémon that can switch in on any of the countered's attacks without taking significant damage and in return reliably being able to attack/status-stall the countered to death. Mixed Victini I'll admit is a little less common than purely physical variants, but it does see use. A neutral natured Victini with 80 SpA EVs reaches 256 SpA. With a Life Orb this is enough to 1HKO 4 HP Houndoom with Focus Blast everytime. Jolly Choice Band Victini also KOs -Def nature, one of the most common types of natures for a Houndoom, with Wild Charge after Stealth Rock damage. Adamant Choice Band variants KO neutral Def natured Houndoom with 4 HP after Stealth Rock damage, as well.
Typically, a savvy player will take note of the foe's Pokémon before the battle. If said player has Victini on their team they'll especially want to take note of any possible Flash Fire Pokémon, in this case Houndoom. Now, seeing as the smart thing to do when facing a team with a possible check/trapper to Victini is to not V-Create blindly, you can expect to be faced with a U-Turn when switching in Houndoom a great deal of the time. Jolly Choice Band Victini 2HKOs neutral Def natured 4 HP Houndoom 100% of the time with U-Turn. Adamant Choice Band Victini 1HKOs negative Def natured Houndoom a little over 50% of the time with SR damage(with U-Turn). Unfortunately, not even Choice Band Max Attack Houndoom can KO 4 HP Victini on the switch with a Pursuit, so the only way to effectively trap and KO Choice Victini locked into V-Create is to let it V-Create before switching in Houndoom, thus lowering it's defense. Unfortunately, that may mean sacrificing a team mate.
In conclusion checks =/= counters, and Houndoom is a check.
EDIT: FFFFFFFFFFFF- That feel when you get ninja'd...
Another Edit:
http://www.smogon.com/forums/showthread.php?t=3446938&page=17 I saw this a few minutes later... Why did I spend 20 minutes writing this post up and doing calcs? *sigh*