Did I underpitch my starter?

I am at day 7 for a Belgian ale. Fermentation was going well for first couple days with mega Kraussen and then tapered off drastically. I took a reading of 1.028 (down from OG of 1.074). When I made the starter I used a stir plate and ran it overnight with 1 liter wort at 1.040 and two vials of liquid WLP550. I thought that would get me up to the cell count needed but apparently not. So now I have two questions:

1- Should I pitch another vial of yeast to finish the fermentation? I tasted the beer and it's pretty good but I think it would be much better if I got it down to the 1.014 estimated. Would pitching another full vial be overkill?

2- How did I mess up on the starter? I used the stir plate to get good aeration but did I run it too long (overnight)? Does the stirring action prevent the yeast from propagating? I'm starting to think I need to aerate and then let it rest overnight.

Seems to me your pitching rate should be fine (though you probably would have ended up with about as much yeast, and more money in your pocket, if you had gone the other way - 1 vial in 2 liters ). I haven't done any experimentation on my own by any means, but the stirplate is widely considered to be the optimal starter propagation method.

Are you sure both your starting and current gravity readings are right?

Was the yeast fairly fresh? Per the online calculators, the yeast vials lose viability surprisingly quickly.

How did you aerate the wort? In a big beer like that the yeast needs LOTS of O2. IIRC, Jamil's Yeast book actually recommends a second shot of oxygen 12 hrs after pitching for a big beer. If you just shook the carboy there probably wasn't enough oxygen for the yeast to grow fully.

How about the recipe? Did you use sugar in addition to malt?

I suspect other factors are playing a bigger part in your troubles than the pitching rate.

Oxygen might be the issue. I'm confident the gravities are right and I used partial grain with LME to fill it out. I didn't add oxygen, just aerated the wort while adding to the fermenter. Is it too late to get this right now or would I be risking contamination too much by going back I'm with oxygen at this point? I suppose I could let it aerate as I transfer to secondary but I'm worried that would cause problems too. If it's too late to save i'll still have good beer, just not quite what I expected. Any suggestions would be appreciated.

Oh yeah, the yeast was fresh too.

It sounds to me like everything is just fine. Wait a little longer and take another gravity reading and see if it is any lower.

I don't see anything drastically wrong with your starter method except that the object is usually to make a larger starter with one vial. I think you had it backward.

At 7 days it probably has a bit more to go. Probably 5- 10 days at least. I would leave it alone for another 2 weeks then ask these questions.

added: mrmalty.com has a lot of information on making starters.

I definitely don't mind waiting a little longer. Is it too late to aerate?

Too late to aerate...growth phase is over.

You're probably fine, most big Belgians take about 10-14 days to reach FG. I'd leave it alone and check again in a week.