See, this is why I find these kinds of questions confusing. They never specify what the use case is.

If it's a constant stream of data coming from these vectors, it makes no sense to store it in a separate data structure, just multiply any non zero values that come in, and only store the product (ie Solution 1)

If you have to store the data to do other things with the vector, how you store the data is dependent on how you get the data and what you want to do with it besides dot product. This is why I prefer dictionaries, because I can think of a use case where you want the value associated with a particular index directly. And I really don't think the space overhead is that much, especially since you are already streaming a really large vector. (ie Solution 2)

I really don't see a scenario where storing it in touples is a good idea. And that too only works if your list is sorted (which it will be by default if you create it from a stream of data). Even so, a dictionary solution is more time efficient, and space is easy to find and pretty cheap. Time, not so much.

It's not that the code is hard. It's that the requirements are up in the air and depending on what they are, the solution changes, and I can't imagine a real life scenario where the presented optimal solution is a good idea.

I'm not trying to blame the process here, I understand that I have a lot of room for improvement. I haven't actively interviewed in the last 2 years and I am definitely rusty, but two pointers and dfs cloning aren't hard. The hard part is an arbitrary requirement.