The afterlife's a bit more vague in the OT
That's an interesting way you put that, as if you find that the "NT" is vague.
Is that the case?
While I can understand the reasons for this in the case of many, the Bible does provide a clear and simple teaching on both the state of the dead, and "after life'". Perhaps that may be due to having an understanding of the foundation laid, regarding God's purpose.
We could though take a closer look at it.
Many living prior to Jesus entertained a hope of living again - Job 14:13-15
So, it is no wonder that Paul was shocked that some did not believe in the resurrection of the dead. Acts 26:8 Since resurrection of the dead was not unheard of.
Elijah raised the dead. 1 Kings 17:17-24
A man that had died, and whose body touched the bones of Elisha, came to life. 2 Kings 13:21
People were aware that God created man to live forever on earth. Ecclesiastes 3:11; Isaiah 45:18; Psalm 115:16; Psalm 37:29, 34; Proverbs 2:21, 22
So, when Jesus came on the earth, people were hoping in him as the one who would deliver them from the scourge of sin - sickness and death.
Even Lazarus' sister Martha, was looking forward to when the dead would return.
She said... "I know that he will rise again in the resurrection, in the last day." John 11:24
Jesus even demonstrated what persons would likely have envisioned.
Luke 7:11-15
11 Now it happened, the day after,
that He went into a city called Nain; and many of His disciples went with Him, and a large crowd.
12 And when He came near the gate of the city, behold, a dead man was being carried out, the only son of his mother; and she was a widow. And a large crowd from the city was with her.
13 When the Lord saw her, He had compassion on her and said to her, “Do not weep.”
14 Then He came and touched the open coffin, and those who carried
him stood still. And He said, “Young man, I say to you, arise.”
15 So he who was dead sat up and began to speak. And He presented him to his mother.
Mark 5:41, 42
41 Then He took the child by the hand, and said to her, “Talitha, cumi,” which is translated, “Little girl, I say to you, arise.”
42 Immediately the girl arose and walked, for she was twelve years
of age. And they were overcome with great amazement.
John 11:43, 44
43 Now when He had said these things, He cried with a loud voice, “Lazarus, come forth!”
44 And he who had died came out bound hand and foot with graveclothes, and his face was wrapped with a cloth. Jesus said to them, “Loose him, and let him go.”
In all these cases, people welcomed backed their loved ones, who fell asleep in death, into their loving arms.
Thus, people living prior to Jesus, hoped to live again, or and see their dead loved ones, in the "land of the living" of which they knew.
The Jews then, knew quite well that living again was a reality.
It was a basic, and simple truth.
What corrupted it, was religious leaders.
The Sadducees for example, taught that there was no hope for the dead Acts 23:8, and so, many people were confused, especially with the warped beliefs of both the Sadducees, Pharisees, and Scribes, who taught manmade doctrines.
By extension, the miracles of healing the sick, lamed, blind, deaf, etc., showed people what they could expect once they are living during
the resurrection on the last day.
Mark 2:9-12
9 Which is easier, to say to the paralytic, ‘
Your sins are forgiven you,’ or to say, ‘Arise, take up your bed and walk’?
10 But that you may know that the Son of Man has power on earth to forgive sins”—He said to the paralytic,
11 “I say to you, arise, take up your bed, and go to your house.”
12 Immediately he arose, took up the bed, and went out in the presence of them all, so that all were amazed and glorified God, saying, “We never saw
anything like this!”
and I don't recall if the NT makes explicit the eternal destinies of any OT figures, or those who died before Jesus in any case.
You are right, there.
The hope of the "OT" figures are not explicitly mentioned... Except, the Bible does make it clear that they will live again. Daniel 12:2
And many who sleep in the dust of the earth will awake, some to everlasting life, but others to shame and everlasting contempt.
To know their destination, one has to understand God's purpose, which was made clear from the beginning, Genesis 1:28, 29, and was later referred to. Matthew 5:5; Matthew 6:10; Ephesians 1:10
The mystery revealed to the apostles, made things even more clearer Mark 4:11, Romans 16:25-27; 1 Corinthians 2:7; Ephesians 1:9, 10, as it revealed how the way to heavenly life was opened for a small number Luke 12:32, who would rule over the earth Revelation 5:9, 10 - thus fulfilling God's afore mentioned purpose. Matthew 6:10
It takes, humility to be willing to be taught, so that one can come to an understanding of these things.
When this quality is met, we come to see how simple this is, while it remains hidden from the masses who do not submit to God's program of revealing truth. John 6:44, 45