To clarify the dragon as is currently:  
1)  Forgotten Relms setting
2)  Cohort to a 13 level Cleric Of Sharress, female PC, using Dragon Cohort feat (would be a too big of a cohort otherwise.).
3)  Male young Brass dragon.  This makes him medium, Chaotic Good, 10 HD, equal to a 1st level sorcerer with cleric, chaos, knowledge spells added to his class spell list.  +9 natural armor.
4) Worships Tamara (dragon goddess, NG, of mercy and healing) as his primary god, Bahamut (Dragon God, LG, of good and wisdom) as a secondary god he reveres.
5) Spells known: 0th level: Read Magic, Detect Magic, Ray of Frost and Disrupt Undead; 1st level: Mage Armor and Cure Light Wounds
6) has a ring that gives him Assume Form that other good dragon races enjoy.
I chose the spells based on his 19 AC and his worship of Tamara.
The DM is allowing up to 2 Flaws from Unearthed Arcana, to gain up 2 extra feats.
The vampire he got trounced by was CR 20+ Githyanki vampire, equipped like a PC.  The Party average level is 14 with 8 players.  The vamp using a 2-handed sword did 3/4 of his hit points in one round.  Yes, the dragon used Mage Armor before physicality attacking and after he use his breath weapon to no effect.  The vamp had a item that protected him from fire.
It did not help that the party was camping and only 1/2 the PC were present.  My PC, the Paladin of Sune and Gnome Wizard were back at the Paladin's barony and the Dwarf ranger/deep warden was the trade caravan we had been with.  We left the caravan to investigate a very large fallen meteorite that passed over the night before and landed "close" to us.