I bled/spotted for 3 weeks post erpc and was getting the faintest 'bfp' still up until about then too... when you say you're still getting positives, pebs are they very faint or at least getting fainter or still quite clear/strong?
For me, I took the tests every 3 or 4 days and could see them getting lighter. That kinda reassured me, as much as the bleeding/spotting was going on for weeks, and as much as I was getting positives, they were moving in the right direction.
If you notice a funny smell, pain, or start to feel flu-ish/ill then getting checked becomes a priority but I would be waiting it out a little more in the absence of any of that and if my tests were getting lighter.
Bear in mind that your loss occured at a time when the hcg starts to peak and also that you had two in there! So you can expect it take a little while longer than 'average' perhaps?
Now, are you really asking me whether you should take the high doese B vits? 
My automatic answer to ANYONE ttc would be a resounding yes! If only because of the improved egg quality (like, duh, who doesn't want their eggs to be the best they can be if they're looking to fertilise 'em?) but also because of the other positive effects it has on cycle/hormone balancing, stress levels, improved hair/skin/nails...
Get thee to Holland and barratt, woman. 
That applies to all of you ttc!
mumtum lovely news! Although, just to check, you did test until you got a negative following your loss, right? As you can see with pebs and my experience, it's quite possible for everything to be 'fine' but you still get a faint bfp for some weeks...
My ex has a place in Barbados (it's his permanent residence although he's only there three months of the year - a tax dodge thing (and before that makes anyone crazy, he's French Canadian so not 'evading' here 
)) . He says I should go... so maybe I will one day, albeit with at least two littlies and DF in tow!