Unfortunately i have to correct you. Denmark lost it's feud over Pommern-Wolgast in 1354, after the Rugischen succession war. In the same year Pommern-Wolgast was integrated into the HRE and got to be a feud of the Empire. So, to be correct neither of the pommeranians was a danisch vassal in 1399. Unfortunately nothing of this is mentioned in the english wikipedia (but in the german one), so i can't prove it to you. You could take as prove the in 1360/61 the Hanseatic league got to war with denmark (see
Treaty of Stralsund) and started their fleet from Pommerania (Stralsund).
In fact, in 1399 Pommerania was partioned in Pommerania-Stolp(Hinterpommern) , Pommerania-Barth (Vorpommern), Pommerania-Wolgast(Wollin) and Pommerania-Stettin (Stettin, Uckermark). In 1397 Eric of Pommerania-Stolp got King of the Kalmar-Union, but he doesn't rule Stolp in this time, so there is no union or something. Actually in 1399 the Danish crown just had no rights in pommerania.