smig. the way you describe your query is what needs clarifying. What do you mean by field?
if you mean a control on a form, or a field in a table, then you are dealing with a complex object, and you can assign [mycontrol] = null (not vbnull, which galaxiom explained is something different)
if you mean a variable, then you cannot assign null to any variable, because variables are typed, and any value has an assigned meaning. The exception to this is a variant, and I presume that a variant is once again a complex object, rather than a true variable,
vbnullstring is a constant that returns whatever constitutes a zero length string.
saying
mystringvar = vbnullstring is tantamount to saying mystringvar = ""
Except that if access/vba was to decide to store strings using a different paradigm, (eg c strings, and pascal strings are structured differently. Are vba strings the same as c strings?) then the assignment using vbnullstring would still work, (because MS would fix vba to make sure it did still work), but a direct assignment conceivably might not.