I have spent the last 30 days trying to fix Dexion  Vampire feed and vampire feeding standing up, modded game... Failed every time  . I have down loaded Skyrim three times and all DLC's , each time Dexion failed to turn to a vampire or played from the dawnguard side ... The dawnguard would attack him .So I moan at Betheda with numurous images  , in the end I asked them to work out why  Vampire feed  standing would not work . I suspected that The download ISO file on one of the servers may be corrupted TE5edit reported corruption . This would happen when I worked for a big ITcompany . Now I have down loaded  Skyrim again today run played only Dawnguared  up to feeding and checking he is at the castle he is , ....... Hey WOW...Vampire feed standing is now working . 
All  FOUR Saves, are Skyrim all DLC's only  Note >>> Also ( NO MODS USED ) .
Saves are up to  Level 7 .I have updated a few perks . Full Vampire armour .Full Dawn Guard armour . A bit of gold .weapons 
I have only used carriges to get around no Town's visited or any quests started .
You also have a horse .
I have used Female Nord .
I have disabled all saving except travel set to AUTO . That is why there is just four saves.
I only found one fix from 2012 and still this has been a problem till today  ,It seem,s  that no longer works very little on the net . So I thought I would do my saves for anyone else who has been having this problem .
I do not know if I can now start modding , Thats my next step . I see no reason why  not if you install certain mods and do not activate them , Or load then back track to Helgan letting the older saves install  go through loading each save to install your Mods I have done this in the past . I will be trying to do this . This is the first time I have up loaded a file to Nexus . As  I see it DawnGuard becomes un playable you can not progress any furthur . If you are also having the same problem I and others have been having , these saves will get you past that Vampire feeding bug . I hope they work for you .Thanks all .