Answer: Two common reasons for the tomatoes to rot on the vine include blossom end rot (BER) and sunscald. Blossom end rot begins at the bottom of the tomato opposite the stem and is a dark rotten area that spreads. Alternating between wet and dry soil conditions can cause BER. This can usually be avoided by keeping the plant well watered so the soil stays evenly moist. Using an organic mulch will also help keep the soil more evenly moist.
Sunscald shows as a pale area on the outside of the tomato and is caused basically by sunburn. This can happen in a hot dry year if the plant is pruned and the tomatoes are suddenly exposed to brutally hot sun as a result. If you recently pruned your plants (pruning is optional but many gardeners like to prune) you might want to shade the tomatoes and see if that helps.
If neither of the above seem plausible, you might take a sample showing typical damage and consult with your local county extension. There are many different diseases that can cause the fruit to rot, but they are difficult to diagnose long distance.
I hope this helps you troubleshoot.
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