Does multiple Forecasters on the field trigger Bison multiple times?

Yes, multiple Forecasters on the field will cause Vision to trigger multiple times when you play a Mech.

According to RiftJudge FAQ #84:

• Ruling: Multiple Forecasters do stack, so playing a Mech with multiple Forecasters in play will trigger Vision multiple times.
• Sequence:
  1. Each instance of Vision triggers separately.
  2. When each trigger resolves, you look at the top card of your deck.
  3. For each trigger, you choose whether or not to recycle that card.

Nuances:

• If you do not recycle the top card and no other game actions occur between the resolution of the triggers, each instance of Vision will see the same card.
• This allows you to look at the same card multiple times in a row or, if you choose to recycle, cycle through multiple cards in your library.