What is the best way for me to get to La Vega or Jarabacoa from Las Terrenas?
Will you have a car? Do care what you will see? Do you care about the time it will take?
I can see three different routes but have only traveled one. You could go along the coast to Puerto Plata and then go inland through Santiago to Vega and on up to Jarabacoa. Your could go via San Francisco de Macoris to the main highway and then go West to Vega/Jarabacoa, thus avoiding Santo Domingo. You could go to Santo Domingo by the expressway (or the slower old road ) and then go to Vega or Jarabacoa.
All three of these routes can be done by bus, but the connections might be a challenge especially if time requires you to change bus lines. (For example, the Caribe Tours terminal in Vega is on the highway a taxi or motor bike away from the place to pickup vans that would take you up to Jarabacoa (60 pesos) if you could not practically connect with one of the Santo Domingo-Jarabacoa buses.
A couple of weeks ago I essentially took this last route on Caribe Tours but I was leaving from Samana. It was about 300 Pesos to Santo Domingo and about another 300 Pesos on to Jarabacoa. It took about 2.5 hours via the expressway to Santo Domingo and about 3 hours from Santo Domingo. I had about an hour connection time in Santo Domingo. Caribe Tours has a website with schedules. This is probably the fastest, most reliable for time and most comfortable way to go using buses. You are unlikely to save significant money using lower class transportation even though it would be slower. (For example, the difference between the lower class bus of another company that made many more stops and the Caribe Tours bus from Samana to Santo Domingo was only 10 pesos (ie. 22 cents US).
PS: If you like coffee buy some Monte Alto grown in the Mountains of Jarabacoa. You will not find it in US and will have a hard time finding it in Santo Domingo. I prefer the whole beans because they last longer. You can get it in grocery stores in Jarabacoa, but you might want to visit (tour) the "factory" and buy it there although the price is the same.
Good luck, bob