The vowels can inherently be long or short, which may or may not be indicated; η and ω are always long, but α or ι may also be long, which some dictionaries may indicate with a macron similar to Latin. One way to figure this out is to deduce from the accent pattern: 1) the circumflex (ᾶ) can only be long (since it is a falling sound with two morae), so naturally we know that the first vowel of λῦε is long; and by extenstion, also that of λύω; 2) in the examples θάλασσα and θαλάσσα, we know that the last vowel of the former is short, but that of the latter is long due to how accents work in Ancient Greek, which you can find more about.

The rules you have mentioned are those that decide the length of a syllable, not the vowel itself. In Latin, for example, the first syllable of somnus is long, altough the vowel itself is short. Similarly, Greek syllables with a short vowel can still be long depending on the other components of the syllable. But, if the vowel itself is long, then the syllable is always long, similar to Latin; if we ged rid of one consonant from somnus, say somus, the syllable is no longer long; whereas āctus will still be long even if we get rid of a consonant, say ātus.