Let me preface this by saying that I really am having a hard time imagining pumpkin in a smoothie but that is a cultural difference thing. :-)
In making a smoothie, I wouldn't worry about the vanilla essence at all and just use the plain yoghurt. I find the sweetness of the fruits/veges, along with all those other ingredients is sufficient to not require the extra taste of vanilla. But if you really feel the need for vanilla essence, only a couple of drops as vanilla can be very overpowering if used excessively. It is only being used as an enhancement, not a primary flavour.
Too many restaurants, not enough time...