OEICs only trade once per day, but do so at NAV. ETFs trade all the time, but trade at small premia/discounts to NAV, so there is a risk you may end up paying more than the NAV when you buy and selling at a discount.
This is no longer true for Vanguard UK OEICs. Vanguard UK has adopted "swing pricing" to replace dilution levies. If the number of units bought exceeds the number of units sold on a particular day, the price of the units is increased to cover the cost of buying the underlying assets. If the number of units bought falls short of the number of units sold, the price of the units is decreased to cover the cost of selling the underlying assets. Vanguard UK bond funds used to have an initial charge, but this has now gone. The cost of buying and selling is now recouped by adjusting the price. OEICs are becoming more like ETFs.