Show that a subset of a set of linearly independent vectors is linearly independent
Students sometimes speak sloppily; often it doesn't matter much but here it seems to me the whole problem is caused by using the language imprecisely:
There is no such thing as an "independent vector"! There is such a thing as an independent set of vectors. If it were true that a set $S$ is independent if and only if every element of $S$ is an independent vector then it would be completely trivial that a subset of an independent set is independent. But that's not the definition...