Your example is what I believe is called Sentence Case, not Proper Case/Title Case. Proper Case (corrected for Roman Numerals) would have been "Class III Wetlands", not "Class III wetlands".
For Sentence Case to capitalize just the first word of a sentence you would normally just use in VB.Net:
' Split string based on periods
Dim s As String = "CLASS III WETLANDS. CLASS IV WETLANDS."
Dim sentences As String() = s.Split(New Char() {"."c})
' Use For Each loop over sentences to change case
Dim sentence As String
s = ""
For Each sentence In sentences
s = s & UCase(Left(LCase(sentence), 1)) & "."
Next
s.Replace("..", ".")
The output of string s would be: "Class iii wetlands. Class iv wetlands."
Of course, that does not work for sentences that include Roman numerals, the pronoun "I", proper names or sentences that trail off with "...". If you want case sensitivity like you get when you use Word, that level of sophistication will not be achieved using simple coding techniques and is beyond the scope of what I could help you with.