***** NOT A TEACHER *****
Hello, Arctica:
I immediately thought of your thread when I read this quotation last night:
"[T]hey had probably bulletproof vests."
The speaker is not a native speaker of English.
If he had had time to carefully write this sentence (instead of saying it), then maybe / perhaps he would have had time to remember the rule: Usually, one should NOT put an adverb between the verb ("had") and the direct object ("bulletproof vests").
He would probably have written: "They probably had bulletproof vests."
P.S. As you know, there are often exceptions to the rules:
a. I believe that his sentence would have been acceptable if he had paused before and after the word "probably" when he said that sentence.
b. In quoting him, then, the written sentence might have looked like this: "They had, probably, bulletproof vests."
c. Those two commas (pauses in speaking) show that the speaker suddenly thought of the word "probably," so he decided to drop the word into the sentence as something extra and not absolutely necessary. We call a word used like that a parenthetical element.
Source of quotation: Time, January 19, 2015, issue, page 17.