Is it legal in Ontario for a company to force you to come off of your vacation early?

Yes. It's also legal to fire you for refusing.

It may be a violation of a union contract and then it's a matter for the union to deal with.

It is legal for your employer to tell you to come in, as a condition of remaining an employee.

In the case you describe, there is ZERO evidence of 'force'. The called and he agreed to come in.

No it isn't. What would they do if you were on vacation in Europe? When you are on vacation you don't even have to answer your phone.

If he is away somewhere and has paid to go, the company has no right to make him leave the place to go back to work. If he is sitting around at home over Christmas, the company could request that he come back early and that would be reasonable, but he does not have to comply. 

If they just left him a message to come back, he should just ignore it, come back when his vacation is over, and say 'what message?'. 

Force is illegal.  Asking is not illegal.

If a company requires the services of an employee that company in Ontario can fire the employee if he does not provide the services.  Check the law.

Your coworker decided not to come in to work - he could be fired.

Was this person "forced" or "asked"?  Big difference.  I think that it is indeed illegal to force someone to forgo their holidays but I don't think asking is.