Joanne, I made the same "mistake", and found that they wanted the present continuous "I'm going to the restaurant later". For these "near future" cases, I'd say translate the French present into the English present or present continuous where possible, as English has the same phenomenon of using the present to talk about the future.

I totally understand your confusion, Joanne. There are three ways of expressing something in the future in French. Ranked acoording to increasing "future-ness" they are:

Present tense: Je vais au restaurant un peu plus tard. -- I'll be going to the restaurant a bit later.

Furue proche: Je vais aller au restaurant bientôt. -- I'll be going to the restaurant soon.

Future simple: J'irais au restaurant demain. -- I will go to the restaurant tomorrow.

As you can see, the English translation isn't always a 1:1 match with the French.

In your example, Je vais au restaurant plus tard, the present tense is used, indicating something that is not far in the future at all. Therefore they were probably looking for an English sentence which doesn't use the standard future tense.

Perhaps it was marked incorrect because you translated "vais" as "come" rather than "go."

While Ron did give good guidance, it's really not done this meticulously in practice though it sounds good in theory.  In practice, there seems to be overlap between the two and there is no official rule specifying a time frame when you have to switch from one to another. Every French speaker seems to decide for themselves. If the action is not pretty much immediate (like screaming "I'm coming!" as you run towards the ringing doorbell) but within 24 hours, both the future proche and the present seem acceptable.

Of course there is a subtle difference. 

Demain, il arrive à dix heures.---Tomorrow's he's arriving at ten o'clock.

Demain, il va arriver à dix heures.--Tomorrow, he is going to arrive at ten o'clock.

I think your answer would also be valid, but the English version you're asked to translate is: "You are going to buy a new car tomorrow" which hints that you should use the futur proche. If it had been "You're buying a new car tomorrow" then your answer would be correct.