Bt cotton is cotton that also expresses one or more Bt proteins (protein genes isolated from the bacterium Bacillus thuringiensis) that are toxic to certain caterpillars when those caterpillars feed on cotton tissues; they have no observable toxicity to insects that are not caterpillars (moths and butterflies). Bt cotton has revolutionized cotton production because many of the primary insect pests of cotton are caterpillars, and therefore the Bt in Bt cotton controls these caterpillar pests so that synthetic pesticides do not have to be used as often, if at all. Because Bt has toxicity only against caterpillars, many farmers observe many more beneficial insects (such as ladybird beetles, green lacewings and predacious bugs) in their fields than when farmers use synthetic broad-spectrum insecticides, and these beneficial insects can help reduce plant damage caused by other insect pests. The seed for Bt cotton is more expensive than non-Bt cottonseed, because the Bt seed produces a plant that produces its own pesticide; one gets the value of spraying a pesticide without having to spray a pesticide.
The primary caterpillar that is a pest of both cotton and tomatoes is the cotton bollworm (when it is on cotton), which is also called the tomato fruitworm (when it is on tomatoes). Note this is the same insect species as the corn earworm, Helicoverpa zea. However, the adult moth of this caterpillar is a very strong flier, so surrounding your tomatoes with cotton (whether Bt or non-Bt) will probably have no impact on adult tomato-fruitworm moths finding the tomato plants.
Therefore, it is highly unlikely that surrounding tomato plants with Bt cotton will control your bugs on your tomatoes, at least no more than surrounding your tomatoes with non-Bt cotton. There are various ways in which you can control the bugs on your tomatoes without having to spray pesticides:
1) Use organic pesticides.
2) Plant tomato varieties that may have some resistance to some insect pests.
3) Encourage and even supplement the beneficial insects in your garden.
4) Use cultural practices, including actually picking the bugs off your tomatoes.
5) Proper ID of the bugs causing damage to your tomatoes will greatly help determine the best pest-control strategy.