Don't forget that là is only one of the many possible copulas. Thì and có are better candidates in this case, but there's also bị, for example...
I am ill > tôi (bị) bệnh
it is red > nó (có) màu đỏ
you are tired > bạn (thì) mệt
that is a dog > đó là một con chó
In fact, "là" is the copula that is least likely to be omitted. :D