thanks. Anything that increases the rate of evaporation of a system will make evaporative cooling more effective.
Lowering ambient humidity Decreasing atmospheric pressure Increasing ambient temperature (though this one is obviously counterproductive) Increasing surface area of evaporation Choosing different evaporative media Adding air movement/wind
ref:http://www.evaptainers.com/updates/2014/8/6/the-sciance-of-evaporative-cooling This means we may have other options to cope with environment counter affects. So it is true that relative humidity is important but it just decrease the affect will not prevent totaly. It is known that one gram of evaporation of water decreases half celcius of one kg water. Vest is said to decrease 10 to 15 fahrenheight comparing to environment and it must be tested at high humidty also to see graph changing with humidity.
thanks for question. Lets Imagine you need to refresh humidity while working. Since you won't be able to take off PPE the vest must be worn over it. The trick is the contact part of PPE and vest. If it is not isolating coldness already it is enough. To increase affect may be more temperature conductive textile material be used on those parts.
with reference to bbc an indian is procuding wearable cooling material using peltier technology. Maybe someone could help him to add maintaining power with solar wearable material also. Here is the link: http://www.bbc.co.uk/news/world-south-asia-13563592 regards