It looks like reduce is a more appropriate method to use for your case, since you want to create an object out of an array.

something like

 rates.reduce((ratesObj, rate) => {
   ratesObj[rate] = account.vent.find(v => v.rate === rate)?.vat;
   return ratesObj;
 }, {})

Add this and your problems will be solved

.reduce(function(p,c){
    return {...p,...c};
},{})

Your code will look like this after adding the above code

rates.map(rate => {
       return {
         [rate]: account.vent.filter(v => v.rate === rate)[0]
           ?.vat,
       }
     })
.reduce(function(p,c){
    return {...p,...c};
},{})

Try this

array.reduce((acc, item)=>{
    return {...acc, ...item}
},{})

Reduce Method:- https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/reduce

A good video on reduce method:- https://www.youtube.com/watch?v=IfieE2G3bfg