I addition to your fine idea, one could add upkeep not related to gold.
You could have a dragon fighting for you with an upkeep of for example 10 resources a turn (gems or whatever). If you run out of this resource the dragon leaves. Imagine how interested you would be in getting your hands on that gem mine over there
which is exactly why raising and levelling your very own dragon would be so very powerful!!! almost no maintanence! Well, other than the infrastructure to house the beast, all diplomatic implications, having to wait a billion turns for it to grow .. ect. Dragon Age and Level would be separate entities. A lv 10 drakeling left to mainly idle would eventually grow into a level 10 drake, with basic stats and abilities rising on a near exponential level. So dragons can gain in strength by both levelling AND simply getting older! oh my!
-> on the note of vampires, another possibility is to give the vampires a manor or two in a certain rural district, or large city, so that during peace-time they can live in the manor and provide for themselves, although during war-time of course they would march with the army, perhaps with their own supply of blood-pets/ cattle. During peace-time however, you would have random events that lead to public outcry against missing persons, each time slowly and subtley pointing toward the vampires. You could ignore it, relocate the vampires (at a higher cost/ aka buying another mansion for these guys), rule with an iron fist/ let the vampires instill fear into the populace. But each time you approve of the vampires' actions and mistreat your populace there is an increased chance that the city/ rural town might rebel.
Of course the lower the population the lesser the chances, and if the vampires are simply too powerful then the mobs would indefinitely be dis-inclined to act on their own but knowing that their tormentors are in fact their keepers will significantly lower their production/ efficiency. In such cases, nuetral heroes of good or neutral cause could travel/trek to seek out the vampires in order to slay them and/or lead angry mobs against them and theoretically against you. Also, evil adventurers could travel to seek succor with the vampires, in hopes they will recieve the dark gift. A small chance they will join your permanent forces, and a large chance they will join the vampire band and increase the maintanence cost.
With Vampires, I think there should be at least one living vampire dungeon as well, which would be some nuetral rural estate most likely, or some darkened castle hidden in the mountains with a population of mostly over-fed cattle (human blood-pets) ... in which case Neutral adventrurers may ask for your aid (possibly joining you FOR FREEE if you slay the vampires) or simply charge in headfirst, which would most likely end in extra experience points for the vampires. In addition, they could be slowly growing in numbers and power if sufficiently powerful evil adventurers travelled in seek of the dark gift. Of course, if you were an evil king that promised even more disirable power or reward, those same evil adventurers could be going to you to seek succor, instead of with the vampires.
Pushing my own agenda, I think there should be a few rare adventurers that prefer the temporance of Dragon religion, or the Cult of Dragon religion, and act more favorably towards civs of the Reptilian Homage civ-trait.
On Temporance of the Dragons, its a goodly religion that accepts all draconic races equally (or biased to the Draconic race that founded the religion, probably Silver, since they are the goodly dragon that has the best balance of raw magical talent and succor with humans)
On Cult of the Dragon, there could be as many as 8 different cults, probably the most common being Cult of the Red Dragon, due to its highest ranking on the evil alignment, followed by Cult of the Black Dragon. The largest cult among the metallic dragons would probably be Cult of the Golden dragon. A cult is a feverish, evil aligned, near obsessive religion, dedicated with extreme bias towards one draconic archetype.
A Cult of the Red dragon would viciously hate all other dragon types, especially the Silver draconids. A follower of CotRD would probably wish to bring back a bloodied, dying silver Wyrmling as a proper sacrifice for some meaningless ritual honoring the legacy of Red Dragons. Its brutal, grotesque, bloody, obsessive, basically an all out Cult of the damned, only filled with Dragon Worshippers.
Although Cults are not always so vicious, there are also Neutral views on Cult of the Dragon. Such nuetrals are usually based upon cult of metallic dragons, although there are rare but existing neutral cults of chromatic dragons.
The most common Nuetral Chromatic cult is the Cult of the Green Dragon. Being closely attuned to the forests, this Acidic dragon usually has the most preference of the Chromatics with direct interference in human affairs, having been known to rule their own kingdoms of men. If a sovereign was looking to make a soul-pact with a dragon, they would most likely look towards a worldly, warmongering Green Dragon for companionship and guidance.
Dark Sorcerers would be more likely to make a soul pact with a Black Dragon, merely because Swamps are the best location to find reagents, as well as to train and find well-preserved undead corpses. Such minor business dealings can sometimes end up with strong diplomatic connections between Sorcerer and Black Dragon, that could eventually turn into a blood pact/ blood oath, and eventually a Soul-pact.
Living Lairs would most likely be White, Green, or Red Dragons, while rarely a possible Silver dragon. Usually Silver Dragon-lairs would only attack evil civilizations, so attacking them is generally up to Evils or Neutrals.
Either way, once you defeat the Roosting Dragon of a living lair, you can either hire it or if youve already killed it, you can take the egg and raise a much, MUCH more loyal, and potentially more powerful dragon compainion. The downside is the long investment plan, and the upside is he/ she will potentially be a permanent part of your burdgeoning empire. A loyal life-time ally, as immortal as the sovereign himself (can live as long).