I'm sorry to answer so late, I was traveling,
But good if you have a better result.
If your extruder is calibrated accurately, you should have the extrusion multiplier in slic3r close to 1. Basically, if you need to push it down to lower than 80 or upper than 1.20, it means your extruder needs a better calibration.
With my SD3, and an E3D hot end (that doesn't change the extruder calibration) I found my E was around 109 step/mm in the firmware. Just to give you an idea. But every extruder acts differently, that's why it needs to be calibrated to your specific machine.
For the Vref, the best way to adjust it is by ear. You set it to extrude, and while it does it, you turn your Vref down on the stepper (with a ceramic screw driver better, but if you don't have one, just use a steel one, and make sure you insulate your fingers from the body of the screw driver), and then turn up until you see the stepper moving without missing steps. you'll see it runing smoothly. If you go too low, it will miss steps, if you go too high, it will get very noisy ! You'll feel it very well.
If you can post some pictures of your prints, to help.
Take care, and happy printing.
(avec ton pseudo, je comprend que tu dois être français, si tu veux, on peut passer sur la section française pour continuer. C'est plus facile de comprendre dans la langue natale des fois !! A bientot!)
SD3, early 2013. Silicon 200W 8x8 heater bed, lawsy carriage on 8 mm hardened chrome shafts, E3D V6 and lawsy MK5 extruder, ATX 350 W PSU, Custom build plywood enclosure, azteeg x5 mini smothie firmware, with vikki 2 screen, fishing line conversion and pulley conversion, M5 Z rod. Calling it Gran'ma, going strong ! <3