String data manipulated given CSV variables
This code is a decent start, as it uses ES6 features like arrow functions. However it could take advantage of many Javascript features - e.g. IsExpenseItemAcceptable could be reduced to a single line using Array.includes(). Additionally, the index variables can be eliminated using the ES6 feature Destructuring assignment - specifically array destructuring.
Instead of lines like

var expenseItemized = expense.split(',')

if ((categoryIndex &lt; expenseItemized.length) &amp;&amp; IsExpenseItemAcceptable(AcceptableCategories, expenseItemized[categoryIndex])){


Destructuring assignment can greatly simplify this to something like:
const [vendor, date, item, price, category] = expense.split(',');   
if (category &amp;&amp; IsExpenseItemAcceptable(AcceptableCategories, category)){

Not only is that condition shorter, it doesn't require the use of the index and is more readable (and a typo on the word &quot;vendor&quot; was fixed).
In the example above const was used instead of var. It is recommended that const be the default keyword for initializing variables. If assignment is required, then use let.
The function GetAcceptableCategories could be simplified using Array.reduce(), similar to Array.forEach().
The data structure could be changed from an array to a plain object - i.e. {} to provide a mapping of vendor and date combinations to prices. This would allow the elimination of the functions updatePrice and IsVenderDateIncluded because the loop could be simplified to:
if (category &amp;&amp; IsExpenseItemAcceptable(AcceptableCategories, category)){
    const vendorDate = [date, &quot;: &quot;, vendor, &quot; - $&quot;].join('')
    if (vendorDate in AcceptableExpenseTotals) {
        AcceptableExpenseTotals[vendorDate] += parseFloat(price);
    }
    else{
        AcceptableExpenseTotals[vendorDate] = parseFloat(price);
    }
}

This would require reformatting the output - e.g.
for (const [vendorDate, price] of Object.entries(AcceptableExpenses)) {
    console.log(vendorDate, price);
}