Transposing and merging cells if certain criteria true

Attached is an example of the spreadsheet I am working with. The "address and vendor id example" tab shows what I need the macro to do.

Essentially I am looking for the macro to do the following:

1) If there is only one instance of the vendor name, leave the row as-is.
lines 4,5 and 6 from the example tab are examples of one instance of a supplier name, need to leave these rows as-is
lines 3, 4 and 12, 13 are 2 instances of the same supplier name, the macro will have to modify these lines

2) If there are 2 instances of the same supplier name with the same mailing address 1, I would like to combine those 2 lines into one. The macro will need to transpose the ID's horizontally and then merge into one cell with a comma and 2 spaces separating them.
lines 3, 4 and 26, 27 from the "example" tab are a good example of this

Also if there are 2 instances of the same supplier name and the mailing address 1 is filled in for one row but now the following row, I need to copy the address from above.

I know this is somewhat unclear but hopefully the examples i did in the "address and vendor id example" tab will help clarify.

I haven't come across a situation yet that this website hasn't solved. Thanks for all your help!

Jeff