Jastrow vocalizes it as יוֹמָא, as in Unkolos, Bereshis 39:11:
הֲוָה כְּיוֹמָא הָדֵין וְעָאל לְבֵיתָא לְמִבְדַק בִּכְתָבֵי חוּשְׁבָּנֵיהּ וְלֵית אֲנָשׁ מֵאֲנָשֵׁי בֵיתָא תַּמָּן בְּבֵיתָא׃
And as for the discrepancy in pronunciation...
Our spelling is based on the Targum for the Hebrew "Yom," day, everywhere that it appears (e.g. Bereishis 1:5; 6:5, etc. I'm not sure why it became Yeshivishly accepted to call it Yuma. I don't know that the Targum ever uses "Yuma" in any context. If you find differently, please let me know. In general, the Targum just adds an Alef to the end without changing the pronunciation: Tzom/Tzoma, Rok/Roka etc.