When are we most vulnerable to the devil?
This is a great question, but I think that the answer could be very different for different people based on different factors.
This is definitely a possibility. Along these lines, Paul said:
Eph 4:26
Be ye angry, and sin not: let not the sun go down upon your wrath:
Eph 4:27
Neither give place to the devil.
Unresolved anger or wrath can certainly give the devil access to our lives. I rarely fall for this trap in the long term anymore. Sure, momentarily at times, but I have kind of figured out the devil's tactics in this regard. I do believe that there is a place for genuine anger or what we might call righteous indignation in a Christian's life, but, even then, we must deal with it in the proper manner. When it comes to me being unjustifiably angry, which does happen, that is always an indicator of something wrong inside of me. Simply put, it shows an area where my own will is not fully subjected to the will of God for my life, and I need to repent/change accordingly, and I ultimately do.
This seems to be my biggest challenge. In fact, just yesterday, I was lying in bed while this thought kept going through my mind:
If I did not know better, then I would swear that I am cursed.
The things that happen to me on a regular basis are so bizarre that they are almost mathematical impossibilities. In other words, in my mind, there is no way that they are just natural occurrences. Something is definitely occurring in the spirit realm, and I am left to ponder if I am dealing with chastenings from the Lord or spiritual attacks from Satan and demons. Sometimes, it is the former, but I normally submit to God's chastenings immediately. The latter? If I sit and ponder long enough, then it becomes apparent to me that the bizarre things that I mentioned earlier definitely increase in regularity and intensity when I am truly seeking after God in order to fulfill his will for my life.
Do such attacks make me vulnerable to the devil?
I do not know. Probably in the sense that it causes me to stop and wonder about things, but probably not in the sense that I ultimately give him a real permanent place in my life.
I hope some of that made some sense.