The question of whether a word is an adjective or not is not always clear-cut. There are several tests which can be applied: if all of them pass, the word is clearly an adjective, while if all of them fail, the word is clearly not an adjective. But there are cases where only some pass, and then there is a judgment whether to count the word as an adjective or not. An example of a word which passes only some tests is "former" - it can precede a noun ('a former minister'), but it cannot be used predicatively ('he is former') or quantified ('a very former minister').
Certainly, qualification with "so" or "very" is one of the common tests, and if you can use them, then the word is probably an adjective (or an adverb, not relevant here).
I was delighted is usually adjectival (as you say, the possibility of I was so delighted implies that). If there is an explicit agent, it can be verbal (I was delighted by each successive vista that opened out as I walked through the park), but I'm not sure that it could be when there is no agent.