Currency

Iceland uses the Króna, France uses the Euro.

They are not interchangeable.

It will depend on what you will use cash for.

If, as you say, you will be using the Visa for all purchases you won't need any.

That may be tough on an autoroute, for breakfast, a cup of coffee, a taxi ride, a bus fare, etc.

I take it you mean a visa credit card. You will not be able to use a credit card all the time, for small purchases (say under €10 or €20) they may not accept credit cards, and you will need cash. If you plan to pay by credit card, always check they accept it before you eat the food!
France uses Euros, but Iceland does not, it has its own currency, the Icelandic Krona.
You do not need to bring all the cash you need with you. It is cheaper to get cash from a cash machine (ATM) in the country than it is to exchange physical money. Get cash after you arrive, using your normal cash machine (ATM) card.
You do need to warn your bank in advance that you will be using your cards (credit card and cash machine card) in case they panic and block the card.

Do you have a debit or ATM card? If so, use that to get cash as needed. Some people bring some starter euros with them to use until they can get to a bank machine - generally around 100 euros. Generally unless your financial institution charges very high fees, the cheapest way to get currency in another country is from an ATM machine. No one knows how much cash you will need because you don't say how long you will be traveling, the number of people in your "we," or exactly when you will "need" cash as opposed to being able to pay by credit card. France uses the euro and Iceland uses the króna -- for questions like that, Google is your friend.

If you have too much local currency when you leave, use it to pay down your hotel bill. That way you don't have to try to be so exact.

Keep in mind that depending on your credit card, you may have fees added for every transaction, which will quickly add up if you use your card for all meals, etc. On the other hand, if you use your ATM card to withdraw cash every few days - 300-500€ - and use that for your daily purchases, you will have fewer of those bank-added surcharges, and French banks do not charge fees for using their ATMs. Use your credit card for major purchases, hotel, car rental, and 3-star restaurant splurges, and use cash for most everything else. Credit unions usually have lower international transaction fees than banks.

Not all Visa providers have same terms and conditions. My Capital One Visa exchanges at essentially Interbank rate with no extra fees.

I always think it's a good idea to arrive in a foreign country with some of its currency in my hot little hand.
Nowadays, it's easy for your local bank to arrange for you to purchase a couple of days' worth of local currency online - there is a small fee for this, but worth the convenience, especially if your credit card won't work for some reason when you arrive. 200 - 300 euros would be enough for a day or two. Many places impose limits of 10 euros or more for minimum purchases.

Before you leave home, check with your bank to verify the 24-hour limit for withdrawals. If you think it is too low, and don't want to incur hefty charges for multiple withdrawals, ask your bank to raise your limit temporarily.

Don't forget to clearly mention to your bank the dates when you will leave home and when you will return. It's a good idea to do this a week before leaving, and verify a day or two before you get on the plane.