I planted some tomato plants in August in pots with 70% soil and 30% organic vermicompost. Plants started to fruit from mid November, but even today those tomatoes are very small and green (and hard). It has been 1 month and they are not turning into mature ones. Are the plants missing something?
Is 30% vermicompost not enough or should I use some NPK fertiliser?
What is the usual time for tomatoes to turn red and mature?
You should be using NPK fertiliser, preferably a commercially produced tomato food, or at least something with an NPK roughly approaching 10-10-18 for a liquid feed, or 18-18-21 for a foliar spray.
Your average temperatures and hours of sunlight also make a difference - in the UK, in summer, average ripening time from swelling of the fruit to full ripeness probably 2-4 weeks.
UPDATED RESPONSE: No, your NPK fertiliser isn't quite right - if you look at the numbers I gave in the first paragraph, the 'K' part is much higher than the other two figures. This is the formulation for tomato food, in other words, the nitrogen and phosphorus (N and P) are lower than the potassium (K).
Where are you located? What kind of temperatures are your plants growing in? Tomatoes like temps in the 60-80 degree range (60s or better at night, 80s during the day) for best growth. If temps are very far outside this range on either side, it may have problems ripening fruit.
