You could try to get the same functionality with mailItem.Recipients property. It returns IEnumerable. Recipient object have a Name member so basically you could do the following (It's in C# but i think you could figure it out with vb):
string recipients = string.Empty;
foreach (Outlook.Recipient r in mailItem.Recipients)
{
recipients += r.Name + ";";
}
You should get the same result as if you use mailItem.To