We have several products that are purchased in smaller quantities on a regular basis, and  larger quantities with a much longer lead time on a semi annually basis.... 

I am worried that when we place a Larger order: that the forecast will see items being ordered and will NOT properly take the ETA time into consideration .

vendor 1 ETA is 3-4 days

Vendor 2 ETA is 120-150 business days

I am overthinking this or is there a way to make sure we keep ordering from vendor 1 till stock comes in from vendor 2????