So the replacement has to be nominated first. Then they have to confirm them in BOTH the house and the senate. This process can take months. During which time America would not have a VP. This is considered acceptable. If in that time Biden should die, (if he's not done so already actor Biden hahaha) then it goes to the Speaker of the House. Which for now is Nancy Pelosi.

Edit: the VP is usually chosen from a pool of sitting senators, or someone already active, though it doesn't have to be.

So the replacement has to be nominated first. Then they have to confirm them in BOTH the house and the senate. This process can take months. During which time America would not have a VP. This is considered acceptable. If in that time Biden should die, (if he's not done so already actor Biden hahaha) then it goes to the Speaker of the House. Which for now is Nancy Pelosi.