One of the last things we expected to hear in this economy is a retailer IPO. But Vitamin Shoppe, which operates 425 units with plans to more than double that number, seeks to raise $143.8 million.The retailer, mostly owned by private-equity firm Irving Place Capital Management, did not provide a timeline for the measure. The Reuters article says that this is happening as private-equity firms are looking to unload the many companies they purchased a few years back. GNC, the largest specialty player in the vitamin space with over 5,000 locations worldwide, had a good first quarter in the recession, with same-store sales increasing 5.4%. That retailer is also privately held.So is it a good time to go public? Are vitamin chains somehow immune to a recession? Or is this more about a private-equity firm looking at its options?