JD Vance has approximately as much policy control in a Trump administration as Kamala Harris would.

VPs don’t have “policy control” but they do have access to the President and the opportunity to influence them. If Trump wins, Vance will be meeting with Trump regularly, and have the opportunity to try to talk Trump into things - I doubt Vance will always succeed, but he probably will sometimes. Whereas a defeated Harris won’t be meeting Trump regularly and so will lack the same opportunity.

This varies from administration to administration. On the economic portfolio, there is zero indication Vance has remit. (From what I can tell, zero involvement in cabinet decisions, for example.)

The VP is ex officio a member of the Cabinet of the United States, as as such is involved in "cabinet decisions". (Strictly speaking the US Cabinet, unlike the Cabinet in Commonwealth countries, doesn't actually make decisions, it just recommends decisions to the President, who can either accept the Cabinet's recommendation or choose to overrule it.)

While originally the US Cabinet did not include the VP – George Washington was under the belief the VP was a member of the legislative rather than executive branch, due to the VP's role as President of the Senate, and as such did not belong in the Cabinet, hence he excluded John Adams – by the 20th century it became standard practice for VPs to be included in the Cabinet. Every Cabinet in recent decades has included the VP. Pence as VP was part of Trump's Cabinet, just as Harris as VP is a member of Biden's.

Now, the Cabinet is just a tradition, it is not established or required by law or the Constitution – so any President could at any time abolish it, or alter its membership. If Trump wins, there would be nothing legally stopping him from breaking with tradition and excluding Vance from his Cabinet. But I'm not aware of any concrete evidence suggesting Trump will actually do that.

The VP has a high level of access to the President, independently from being the most senior member of Cabinet (the President is not technically a member). Of course, the VP doesn't have the legal right to meet with the President, so if a President comes to dislike a VP, they have the right to freeze the VP out and refuse to meet with them. But again, no reason at present to suspect Trump would do that to Vance if Trump wins. On the contrary, Trump (not without reason) views Vance as a "mini-me", and as such may be more likely to listen to him than to other potential advisors. Many critics accuse Vance of sycophancy, but for Trump that's less a criticism and more of a positive.

Sorry, I meant agency appointments. Not cabinet decisions. That game is in its last inning right now with both campaigns. From what I know, Vance has not been consulted on economic considerations outside crypto.