I mean yeah it's vulnerable in general just having the capital on the coast, but the AI will like send an army to the Dixie HQ to stop an invasion (drawing it away from another front!) but if you cancel the invasion they'll redeploy to a front, giving you a couple weeks to invade unopposed as long as they don't have extra armies (which they won't if they're busy fighting the French or British). There's no penalty to canceling a naval invasion apparently, you can immediately restart it the next day