As far as I am concerned trickle vents are a failed concept. The idea was that as British homes become less draughty you need to introduce deliberate ventilation to shift humidity. The problem is that when the wind isn’t blowing there is negligible air movement but when it is howling a gale outside you get excessive air movement. The draughtiness of a house was limited by building regulations to 10 air changes an hour, i.e. the air you heat in your house gets fully replaced every 6 minutes. Under such regulations trickle vents compare favourably with leaving all the windows open.
The building industry pressurised the government into scrapping any real improvement in the latest building regs as improvement would be “too onerous”. (Compare that with Passivhaus regulations of not greater than 0.6 ach.) If your house complied with building regs, only, then you don’t really need trickle vents. A few years ago I had my house tested and it had 7 ach. I have since made improvements. Despite various window replacements over the years I have never owned a trickle vent and don’t intend to do so. There are currently 5 people making humidity. I do not get condensation or mould in the house, partly due to an effective ventilation system and partly due to even heating in all rooms preventing cold patches.
I would go further than @john.d and say that if using MVHR you should avoid them. In fact any ventilation system that introduces fresh air is disadvantaged by having trickle vents.