I may be incorrect, but I don't think anti-vaccine has ever been an official platform of the part as a whole. However, Jill Stein is unabashedly in the "I'm not saying all vaccines are bad, I'm just saying there's no level of evidence that will ever satisfy my burden of proof that they are safe either", and has been, to varying degrees, on the Andrew Wakefield "vaccines cause autism" bus for a while.
She's very open with this view when among only Greens and when interfacing with the fringe parts of the Autism community, but when trying to promote her campaign or party to the general public she does everything she can to ignore the question and be silent. I don't know if it's an official platform, but the party's stance as a whole has generally been "vaccines are a personal choice only"
For those unaware, Stein is also a physician, which makes this all the more embarrassing and pathetic.
The weird thing is, Bernie has been very obviously and intentionally ignoring her pleas to jump on his bandwagon. It's disappointing his followers aren't taking the hint - the Green party is completely toxic to any progressive agenda because of all the extra baggage.
edit: It's also worth noting that Trump is also anti-vaccine, though his enthusiasm for the position depends on the time of day you ask him (just like all of his positions).