Sumif Vlookup combination?

Hi guys,

Hope you can point me in the right direction and help me with a solution. I have a list of vendors in the "data" tab of the vendor # as well as the sales $ broken down by weeks (1-26).

In my "summary tab, I have certain vendors that we will be onboarding on certain weeks. My vendors in the summary tab are by Months.

What I am trying to do is if vendor A has a go live week of week 6. It will sum week 6,7, and 8 figures and puts it into March (my fiscal week starts in February). Week 9, 10,11, 12, and 13 will be summed up and put into the April field and so on until week 26.

Feb (4 weeks), March (4 weeks), April (5 weeks), May (4 weeks), June (4 weeks), July (5 weeks).

If vendor xxx is going live on week 25 then it will only sum week 25, and 26 into July.

Pl see attached file with formula.

Thank you so much!! This was extremely helpful!!!!

Thanks everyone for your help. If I want to add a little twist to this (trying to improve my model) - how would I go about?

In the data file, I've added a department level in column C. I still want to yield the same results so I would want the summary tab to sum up all Vendor AAA (regardless of department) based on the given weeks (cells to add in data tab highlighted in green)