what is unclear in "He is such a nice young man"?

Question

is it too vague? or is it Equivocation, or is it Ambiguity?

same question for “Converting our cars to hydrogen fuel cells would have a significant effect on the nation’s dependency on foreign oil.”

Answer 1

It is hard to respond without further context.
1) He could be ambiguous, as the reader perhaps didn't know who you were referring to specifically.
Further, perhaps it was vague in the sense that WHY was he "nice".

2) Though it is true, as I think you intended it, "converting our cars to hydrogen...". Perhaps you meant the whole body of cars, as a generation with a gradual replacement of current technology with ones with hydrogen. Because converting each car to another technology would be outrageously costly.