Solid means firm, or definite. If something is definite, then it is 100%. If you want to get into definitions, the word you're describing is "probable." Yes, the chances of him switching is commitment to UNC is low, but there is a chance. If Vereen is announcing his decision between Tennessee and UNC on Friday, that is not solid. He may be leaning towards UT, but he's not a solid commitment. Hopefully, he will be after Friday.
If you use that denotation for "solid" then no recruit for any school is solid because no kid is 100% until they sign the papers. However, the word solid does not have a denotation in reference to how it is used in recruiting. I don't know how you got that solid means definite or firm in the essense of future predictions. Those terms, when used to define solid, are refering to a substance being "solid" in makeup.
In a players' mind, they can be a solid verbal commitment or a soft verbal commitment. Obviously, solid would mean there's no waivering, and soft would mean they're not for sure about their decision. Vereen is waivering. He is considering UNC, and that is why he will announce his decision on Friday. He is a soft verbal as of right now. Any more questions, teacher?