I have had a quick look at it and as a level 8 solo creature it really needs to be a much greater threat.
As it stands it only really has 2 attacks, one is a single target for lower expression damage, and the other is a blast 2 again for lower expression damage.
A solo creature needs to be able to be as threatening as five creatures. This doesn't always mean that it should be able to attack five creatures each turn, but at a minimum it should be able to attack two, preferably three. At least one of these attacks each turn should also be meaningful, i.e. do a threatening amount of damage or result in a serious status effect.
This creature also has a "cloud of fog" power, similar to the black dragons darkness power. In this case it doesn't recharge, but it doesn't have an up keep and can be moved with a minor action. Many words have been written about the black dragons darkness power and how it can seriously reduce the fun of player characters, by adding a layer of frustration. I can see the thematic relevance of the power but I would be wary of it none the less.
The idea of it being able to change form is an exciting one, but it needs to be defined and seperated more. This is an excellent opportunity to add phases to a solo creature. Why don't you have the vampire change form when bloodied and have a different stat block with unique powers? This would keep the encounter dynamic and interesting, keeping your players on their toes. Near death the vampire could change again for a last breath desperate stand, again with different attacks. Good Solo creature shouldn't be just like standard creatures with more HPs, they should be special and have a deadly air of the unpredictable.
As a minion your Zombie spawn probably doesn't need the Zombie Weakness power, any critical hit is going to kill it anyway.
I hope these thoughts help, as always this is just my opinion.