Olive oil in Japanese cooking
Olive oil is not native to Japan and is never used in traditional Japanese cooking.  (Yes, olives are now grown in Japan and olive oil is readily available, but so are burgers and pizza.)

Your recipe's "vegetable oil" is almost certainly a translation of the Japanese サラダ油 sarada abura, literally "salad oil", meaning any of a number of mildly flavored, neutral oils like canola oil (probably the most common) or sunflower oil.  According to the relevant JAS standard, olive oil is explicitly not a type of "salad oil".

That's the theory, but in practice, for things like salads you probably can use olive oil without significantly changing the flavor.  Salads are also not traditional Japanese food, so there's more room for experimentation anyway.  Stick to mild olive oils though, avoid funky extra virgin and the like, and definitely do use a "salad oil" if the recipe involves frying etc.