What vehicle did God use for Jonah to receive this word?

As the text says nothing, anything else is mere speculation.

There may be some traditions/oral history that fill in more of the details (I'm not aware of any), but whether they tell you what really happened depends on what authority you give to each source - in fact whether you believe God spoke to Jonah at all depends on what authority you give to the biblical text and how you interpret it.

But, to speculate, there certainly appear to be times when God speaks to his prophets in an audible voice - for example Samuel believed it was Eli's voice calling him. I think this is fairly likely, but as we aren't told it may have been completely different.

Jonah is one of my favorite prophets, and your question is interesting. Hope this will be helpful to you and others.

When we are trying to understand a particular passage, we can gain a lot of useful insight by using the basic rules of Hermeneutics - it's a 50 cent word for Biblical Interpretation. One of the most important principles is that we always look at the context.

• that is, the historical context, -literary context/ what genre of literature.
• cultural context and
• Grammatical context.

We get some helpful information right away. Jonah is not poetry, allegory, or wisdom literature, or apocalyptic literature like Daniel or Revelation. In the Old Testament, the 39 books are either Historical/Law, Poetry, and Prophets. It was an actual story by real people in a real city, and this is important because you can't take Genesis which is History and make it symbolic - which many people do. The text says that God spoke, God replied.

Another basic rule of Hermeneutics is "If the passage seems to make literal sense, seek no other sense or it will be nonsense." If we are reading historical narrative and it says there was a man on a donkey, then the reader should not imagine that the donkey or the man are symbolic. So our first clue from the genre shows that there is no reason to speculate that Jonah had a dream, or vision. To read something into the text that is not there is exegesis.

Finally, another important foundational principle of hermeneutics is that scripture interprets scripture. What other passages can we use to compare or shed light on this one?

We have many examples of real prophets, who heard God speak in an audible voice and we see this over and over in the Old Testament. Samuel, Adam, and Nathan the prophet talked to God. Daniel, Moses, and Abraham, Moses, and many other prophets or Old Testament saints talked to God. We even have descriptions of God's voice to Moses, and God speaking to the young servant Samuel.

Also, we have many examples from scripture where people had dreams, or visions and heard from God in this way, and it is recorded in scripture. Jacob had a dream, of the ladder going up to heaven. Joseph the Husband of Mary, was warned by God in a dream, and if Jonah had had a dream, then it would have said this.

I love that Jonah is the only prophet that Christ compares Himself to, and we have the sign of Jonah being in the great fish 3 days and 3 nights, and Christ actually quotes the prophet, and the only sign that he gives that He is truly the Messiah, is this sign - that the son of man is in the earth 3 days and 3 nights.

Jonah chapter 4 does not say, but suggests strongly, that God spoke to Jonah audibly while Jonah was awake, so it was not a dream.

4 But to Jonah this seemed very wrong, and he became angry. 2 He prayed to the Lord, “Isn’t this what I said, Lord, when I was still at home? That is what I tried to forestall by fleeing to Tarshish. I knew that you are a gracious and compassionate God, slow to anger and abounding in love, a God who relents from sending calamity. 3 Now, Lord, take away my life, for it is better for me to die than to live.”

4 But the Lord replied, “Is it right for you to be angry?”

5 Jonah had gone out and sat down at a place east of the city. There he made himself a shelter, sat in its shade and waited to see what would happen to the city. 6 Then the Lord God provided a leafy plant and made it grow up over Jonah to give shade for his head to ease his discomfort, and Jonah was very happy about the plant. 7 But at dawn the next day God provided a worm, which chewed the plant so that it withered. 8 When the sun rose, God provided a scorching east wind, and the sun blazed on Jonah’s head so that he grew faint. He wanted to die, and said, “It would be better for me to die than to live.”

9 But God said to Jonah, “Is it right for you to be angry about the plant?”

“It is,” he said. “And I’m so angry I wish I were dead.”

10 But the Lord said, “You have been concerned about this plant, though you did not tend it or make it grow. It sprang up overnight and died overnight. 11 And should I not have concern for the great city of Nineveh, in which there are more than a hundred and twenty thousand people who cannot tell their right hand from their left—and also many animals?”

The evidence is that the Lord caused a miracle - the growth and death of the plant - so the communication was in the phycical realm, not a dream or vision. Certainly Jonah is speaking aloud. There is no mention of a whirlwind, fire, earthquake, smoke, cloud, or other physical phenomena associated with messages received by Moses, Elijah or others. Yet these physical details are give - the hot wind, the sunrise, the tree Jonah is sitting against. Since so many details of the setting are given and no mention of a visible form is described for God, then an auditory event is the most likely. Jonah heard God speak while awake but did not see a vision of Him.

(For completeness, the only other mention of Jonah in the Old Testament is in 2 Kings 14:25, but nothing there seems to shed light on this question.)