What Bitcoin wallet will allow me to participate in an ICO? I have bitcoins stored in an online Coinbase wallet. I've read this wallet should not be used for investing in an ICO as this will be Coinbase's wallet (not mine) and I won't have access to the private keys.

Is this true?

I looked into creating a multi-sig Coinbase vault as this seems like a good and safe middle ground. However, I noticed the vault has a 48 hour waiting period which is not desirable for buying into an ICO.

I'm running Mac OS X. Is Electrum wallet this best way to go about this?

What do people recommend?
Coinbase is mostly used to buy Bitcoins and hold your bitcoins, It is indeed YOUR WALLET, the vault portion of it is actually the best feature they have. You are required to confirm removing the coins often via your phone, and two email address and it gives you 48 hours to cancel on any of the devices.

In my book, this is what a vault means to me, multiple check points before they even move anything out.

Then once they are out they can be moved to any address of your choosing.  Including exchanges like HitBTC which is one I like to sale BTC / BUY ETH for instance, or to another coin or ICO. Most coins I mine can be sold, to purchase BTC and return the funds to Coin Base, and added to the vault for long term.