Well, yes. And they're not just vulnerable to .50/12,7mm and 14.5mm fire, but to 7,62 MG fire as well. When samples of the Stryker's armor were tested against 7,62 MG fire, the Russian RPK, using the AK round had to be used rather than the now-standard PK machinegun, their equivalent of our M60 or M240 MAG. Then they cut back to a 16-inch barrelled AK after the RPK's 24-inch barrel managed to penetrate with Soviet M43 steeljacketed ball ammunmition, never mind the newer AP rounds. Ooops.
The Strykers have external saddle tanks, good from the point of view that they won't roast the crews alive when hit by RPK or larger AT weapons; but unfortunate from the standpoint that fire directed into the middle two tires on either side stands an excellent chance of rupturing the tank and setting fire to the tires on that side. A hit from an M203 HEDP round of Soviet GP-30 underbarrel grenade laucnher will be worse, but it can still be expected that the old RPG-7D and V models will make superb Stryker killers. And since the Stryker is already at the maximum width to be carried in a C130 [crews aboard have to climb over the vehicles inside; there's no room to go around them inside] applique or add-on armor can't be used.
Then there's the little problem of the rollovers the Strykers have had at Fort Irwin. And while the tires are said to be bullet-resistant up to .50 cal, it turns out that 12-gauge shotgun slugs are large enough to deflate them very successfully. And that's before considering some of the really evil tricks dreamed up for use against the things, or the usual line of antitank weapons that are iffy against an M1, but ideal against the Stryker.
-archy-/-