Apologies if this this has been asked before. I am about to start Dawnguard to get rid of or reduce the annoying in-town vampire attacks. I don't want to add dragons into the mix just yet. Any locations where I can safely store items until I am ready to purchase a house?

Try to take a look here: http://www.uesp.net/wiki/Skyrim:Containers#Safe_Containers

Edit: Should have checked the link first. Here is the correct one: http://www.uesp.net/wiki/Category:Skyrim-Places-Safe. Sorry for the confusion.

Anise's cabin outside Riverwood. you'll have to kill her I believe but the storage in her cellar is safe.

In addition, no furniture in the game is set to Respawn, so as long as there is no ownership assigned (red hand) then you can store things in desks, cabinets, dressers and the like in just about any location. There are a few chests that respawn, besides the BOSS chests in the dungeons, there are a few chests in places like guard barracks and common houses that might respawn your stuff, but most other chests are safe.

Just don't be like me, and put things in various places all over the map thinking you will remember where you put everything. I had multiple houses and spent more time looking for things that I put away than I wanted to.

General rule of thumb: wardrobes, end tables and the like do NOT respawn. Chests, barrels, sacks and open surfaces DO respawn. Note that there might be exceptions, so always test first if you're unsure. If you place an item in the container, leave the cell, wait more than 30 days (you can always reload afterwards), and the item still is there when you return, you're safe.

Note that if the container belongs to someone else, your stuff will be marked as stolen, so you can't just store things in a random npc's house.

I think for instance the wardrobes in Fort Dawnguard should be safe, but test first. Same with Volkihar Castle, if you're going that route. Then you have Alchemist's Shack between Helgen and Ivarstead (the end table is safe), Anise's Cabin has been mentioned (the basemant is safe, even sacks and open surfaces). There are other places as well, both dungeons and shacks.

You could also marry an npc with a house, to access their house.

I wonder if Faendal's house is safe. You can get the key from him the first time you arrive in Riverwood.

The Alchemist's shack near Ivarstead.

Join the College at Winterhold and you get a room with a bed and storage.

I just released him as a follower and picked up Jenessa although I was wondering the same thing.