The vertex is an imaginary point, and usually signifies a karmic event in your life, which could be but is not necessarily romantic. A partner's sun/moon midpoint on your vertex is an important link, and if this is reciprocal it would indicate a relationship that is destined to deeply touch both of you.
Whether you two are compatible or not is another question, and the sun/moon/vertex conjunction doesn't necessarily indicate this, you have to look at other factors in the charts.
Certainly, it is very significant and the contact will be very important in your lives, whether you remain together or not.