Is there code that states there must be a sheet of plastic between vinyl siding and the house sheathing?
Yes. What he was talking about was the "water-resistive barrier." (WRB) This is typically Tyvek housewrap (a kind of vapor-permeable plastic) or grade D building paper (tar-soaked paper). All wood-framed buildings need one of these, or else any water that penetrates the siding can contact the sheathing, and will quickly rot it out and infiltrate inside. All buildings in jurisdictions covered by a building code are legally bound by this requirement.

If your condo building was re-sided by somebody who removed the WRB and did not put another one, you have an emergency situation on your hands. You need to IMMEDIATELY have someone remove the vinyl siding, replace any water-damaged sheathing, studs, and drywall, and install a proper WRB behind new siding. This is not optional. If you fail to do this, the building could be rendered structurally unsound and effectively uninhabitable due to mold; the aforementioned building materials are highly vulnerable to water. It will cause them to grow mold, become soft, and get eaten by termites.

Another possibility, because the residents say the water is coming in at the same place in each unit, is that the windows were not flashed properly--especially if that place is under any windows. This is a more common error than forgetting the WRB entirely but no less important to fix in a climate where it rains. The solution to this problem would be to make sure the wall's WRB is properly integrated into the windows' nailing flanges. A common error is not lapping the WRB over the top flange, but rather nailing the flange over the WRB--which allows any water flowing down the WRB above the window to get behind the window's flange and into the wall. Needless to say, if there is no WRB at all, the windows are by definition improperly flashed, and could easily be the water infiltration points.

It is highly likely that you have an actionable legal case against the company that installed the vinyl siding if either of these defects can be proven. Unfortunately, any company that will make such galling errors is unlikely to be well-insured.