In mainstream Iranian Persian, the second vowel is a short /o/, so pāsox.Yes, there are two vowels in this word: pāsux. The vowels a, i, u are always unwritten in Dari.
See this thread. If you understand Cyrillic script, I can send you a dictionary by Рубинчик (1970) that provides transcription. Keep in mind that transcription in dictionaries reflects the Iranian Persian pronunciation, not the Dari one.What is the best way to know which vowel is going to be? Maybe you know some good dictionary for website?
Unlike in Arabic, grammar won’t help much; you just have to memorize the vowels. Arabic script isn’t well suited to write non-Semitic languages.Maybe there are some grammar rules?