From: Ottawa, Canada
Trade agreements among major powers on the same side do not exist in WiF. Instead major powers on the same side announce, during the Lending phase, whether they are lending any resources, oil, or build points to eligible recipients and, if so, in what quantity.
First paragraph of §5:
In this stage, you can announce that you are giving resources and/or lend leasing build points (see 13.6.4) to another major power on the same side this turn. [Emphasis mine.]
Trade agreements can exist between major powers on opposing sides, such as the pre-set 1939 trade agreements between the USSR and Germany or the US and Japan. In WiF:FE, major powers could also agree to a trade agreement as part of a peace treaty.
See first paragraph of §13.7.3:
Two major powers at war can agree to come to peace on any terms mutually acceptable (except for transferring units).
"Any terms acceptable" allows such things as trade agreements. However, my understanding is that MWiF does not provide functionality for major powers mutually coming to peace except for, if the optional rule is selected, the USSR & Japan.
At any rate, a major power can't lend to a major power on the other side.
So, bottom line: Vichy France can never lend resources to China.
If France had announced it would lend a resource to China during the Lending Phase (RAW §5) of, say, the May/June 1940 turn, and committed the resource in Indochina to meet this commitment during the Production Step (RAW §13.6), all would be well for that turn. If, however during the turn the Germans capture Paris, they may establish Vichy France during the Peace step (§13.7). So from that point forward, in the event Indochina became a Vichy-controlled country, that resource would no longer be available to be lent to China.