Keep in mind the "partner" and "vendor" entities are just other
dimensions of the customer entity. They carry with them specific
functionality... vendors can ship you product and you can pay for that
product. Partners can sell your items and you can give partners
commissions on their sales. Partners can also log into your system and
enter data if you allow it (i.e. create new donor records, for
example).
If the business goals is to track the revenue earmarked for a specific
trip, I would suggest this:
Each volunteer is a new constituent (i.e. customer record).
Add a custom check box to the customer record to track the long term
volunteers or just use the volunteer check box provided in SuiteDonor.
(now you can generate a search that lists all your volunteers by
looking for all the constituent records with the volunteer box
checked)
Under the long term volunteer, create a project for their mission to
create their donations to their mission.
If others are donating to the mission, I'd either create a custom
record for the missions -- when you get a donation, you can specify
the mission it is for - or I would create a list of missions in the
"class" list and code each transaction appropriately.
Then you can build a report that tracks revenue and expenses by
filtering the transactions according to the custom field or the class.