What's the pro's and cons of a single file import VS a multiple file import? For instance, a Vendor Bill. Seems like you can represent both the header and line with both method?

If it's how your data already exists use it. If not, I wouldn't bother

For transactions it does not really matter but for entities with multiple addresses, multi-file is easier. With single file, you need to add columns for the second address. You can also not combine records with 2 and 3 addresses because the ones with two address will fail because of a required field missing for the set of address fields for the third address.