Don't parse CSV manually when you have handy good quality libraries available. Please!
CSV parsing has many many potential pitfalls and this library, according to my testing, solves most of them neatly.
That said, if this is a one off task and the strings are always like your example, you can use regex, like this (VB.NET syntax might be wrong, please fix):
Dim s as string = "1, 2, '1,233,333', '8,444,555'";
Dim r as Regex = new Regex(",\s");
Dim re() as string = r.Split(s);
This counts on that there is always a space after the separating comma and that there is no space in the commas between the numbers. If that's not always the case you can:
- Make the regex more complex (look here to see how messy things could get)
- Use the library and be happier