No idea about MC12 (that's a particularly buggy version). But in MC14 the problem is that the READPRN is not being properly evaluated by the symbolic processor, but is being left as an unevaluated expression. You can work around this by using the combined assign/display, which uses the numeric results as the symbolic value.
Your vectorizations are incorrect and meaningless. You cannot vectorize the subscripting operator, which inherently applies to a vector.
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� � � � Tom Gutman