Recently got the Dawnguard DLC. My only complaint is that after the DLC was released I have ad problems with buying decorations for the Breezehome in Whiterun.

If at any point in time I try to buy the Alchemy Lab the guy just takes the money. He doesn't give me the lab in the house or remove the text. What can I do to fix this?

Nothing.

I haven't tried it yet, but others have said if you uninstall Dawnguard, you can get the alchemy lab.

http://www.gamesas.com/topic/1339340-xbox-360-bug-report-topic-v3/page__st__360

(See bottom of the page)