vi: modify phoneme a and rules for "oă"

dictsource/vi_rules

         oái        wa:I3
         oải        wa:I4
         oãi        wa:I5
-        oă         o@1
-        oằ         o@2
-        oắ         o@3
-        oẳ         o@4
-        oẵ         o@5
-        oặ         o@6
+        oă         wa1
+        oằ         wa2
+        oắ         wa3
+        oẳ         wa4
+        oẵ         wa5
+        oặ         wa6
 		oc		   O#k
 		oanh	   we-n^
 		oành       we-n^2

phsource/ph_vietnam

 	IF nextPh(N) OR nextPh(c) THEN
 	FMT(vnasal/a_n)
 	ELSE
-	FMT(vietnam/a, 60)  //a_4)
+	FMT(vietnam/a_2, 60)  //a_4)
 	
 	ENDIF
 endphoneme