ranger silence dekan, dekan die, ranger attack from a distance, further than your skills reach
dhan silence dekan, dekan die, dhans attack from hide, you won't see them comming
guard stun + mana drain dekan, dekan die, guards use rohas blessing/stone skin
warlock debuffs the crap out of you, drains your mana, broken treasure and you die. warlocks can use mirror, intel shield and fake death to out play you easily
Honestly with only 2 short stuns, no cc resist beside dragon skin which is easy to get around, being heavily dependent on not being silenced and having low movement speed. I don't see how vit dekans are anywhere near a top pvp class. At least dragon sage has reflect at +20 which helps heaps to not get perma cc chained, thus why I'd rather go for intel sage than vit knight.
for PVE purposes, sure VIT dragon knight is a nice char to play, you can get cheap armour coz you can tank the mobs anyway but honestly, a guardian or such is a better alternative PVE wise too, at least for me that is.