Vessel currently makes use of automatic fees with a slippage threshold in an attempt to make fees simpler for users to work with.
For all transactions that will incur a network fee, we make a best effort attempt to show an estimate on what those fees will cost in current USD terms, and allow you to give consent or cancel. We use live on-chain historical data in order to make estimates of what fees and gas prices are likely required to give a transaction a high probability chance of being included in a block. That said, due to network congestion and fee volatility in general, it may be required at times to submit your transaction more than once before it will successfully go through by way of being mined into an active block.