Not sure if this is what you want, but think so:
private void RenumberPreferredSequence(Item record)
{
var seq = 2;
foreach (var itemVendor in record.ItemVendors)
{
itemVendor.PreferredSequence = 0;
if (itemVendor.PrimaryVendor)
{
itemVendor.PreferredSequence = seq++;
}
}
}