After writing the above, I checked and learned there is a Caribe Tour terminal in Jarabacoa and that there are buses leaving Santo Domingo for Jarabacoa at 7 and 10 in the morning and 1:30 and 4:30 in the afternoon. Since these buses pass through La Vega, you should be able to get a bus to Jarabacoa from the the La Vega terminal about 2 hours after it departs from Santo Domingo.
The Caribe Tour terminal is on the outskirts of La Vega, the bus from Samana will pass by it. You should tell the driver you want to get off at the Caribe Tour terminal in La Vega.
If you will also need to return to Samana from Jarabacoa, let me know. It will be a little more complicated, but it is doable in public transportation.
It has been about 5 years since I last took the public bus from Samana to La Vega/Santiago, but up until then it was leaving from the Farmers' Market about 7 am in the morning. I would think it would be about the same today. There is also a public bus leaving Samana for Santiago about 1 pm, but I would think you would prefer to go in the morning because this will permit you to travel from La Vega in a comfortable bus instead of a crowded, frequently-stopping guagua.
