How to convert a variable int to vector?

effectively one of the problems is that aux1 is not the number you want, do not parsea well. If you do this, it will parse well: Character.getNumericValue(aux) , apart you have to subtract 1, since in your arrays the numbers start at 0.

  

aux1 = Character.getNumericValue (aux) -1;

On the other hand, in the if you must compare against i. At this point, it would only be necessary to break the number upside down.

An example of operation:

for (int i = 0; i < vec.length; i++) {
        aux = vec [i];
        aux1 = Character.getNumericValue(aux)-1;
        if(i==-1){
            System.out.print(decena_m[aux1]+ ' ');
        }else{
            if(i==0){
            System.out.print(unidad_m[aux1]+ ' ');
            }else{
                if(i==1){
                System.out.print(centenas[aux1]+ ' ');
                }else{
                    if(i==2){
                    System.out.print(decenas[aux1]+ ' ');
                    }else{
                        if(i==3){
                        System.out.print(unidades[aux1]);
                        }
                    }
                }
            }
        }
    }

Your program has several problems:

Here is your functional and improved program without VARIABLES TRASH XD.

    import java.util.Scanner;

    public class Test {

    public static void main(String[] args) {

        String unidades [] = {"uno", "dos", "tres", "cuatro", "cinco", "seis", "siete", "ocho", "nueve"};  
        String decenas []= {"diez", "veinte", "treinta", "cuarenta", "cincuenta", "sesenta", "setenta", "ochenta", "noventa"};  
        String centenas []= {"ciento", "docientos", "trecientos", "cuatrocientos", "quinientos", "seicientos", "setecientos", "ochocientos", "novecientos"};  
        String unidad_m []= {"mil", "dosmil", "tresmil", "cuatromil", "cincomil", "seismil", "sietemil", "ochomil", "nuevemil"};  
        String decena_m []= {"once", "doce", "trece", "catorce", "quince", "dieciseis", "dieciciete", "dieciocho", "diecinueve"};  

        Scanner leer = new Scanner(System.in);
        System.out.println("Escribe un numero: ");  
        char vec [] = leer.next().toCharArray();
        int currentVectorValue=0; 

        for (int i = 0; i < vec.length; i++) {
            // Tus arrays de unidades centenas.. empiezan todos en 1 (diez, cien) 
            // pero el array se inicializa en 0, por lo que le quitamos una unidad 
            // al valor recuperado, EXAMPLE: si recuperamos un numero 1, 
            // vamos al vector unidades[0] ya que en la posición 0 está el valor 1
            currentVectorValue = Character.getNumericValue(vec[i]) - 1;

            // Para saber si es unidade decenas centenas etc : vec.length - i
            switch (vec.length - i) {
            case 5:
                System.out.print(decena_m[currentVectorValue]);
                break;
            case 4:
                 System.out.print(unidad_m[currentVectorValue]);
                break;
            case 3:
                System.out.print(centenas[currentVectorValue]);
                break;
            case 2:
                System.out.print(decenas[currentVectorValue]);
                break;
            case 1:
                System.out.print(unidades[currentVectorValue]);
                break;
            default:
                System.out.print("Out of range");
            }

            System.out.print(" ");
        }
    }
}

I will provide my answer because the others do not take into account when it comes to numbers that end in tens (11, 12, etc. print "ten one", "ten two", etc.)

That is the only case that I deal with in a particular way inside the loop. For the rest, I changed the if's for a switch that I think is much cleaner, apart from using the correct condition that I can use to know which digit I'm dealing with in the iteration.

By keeping a certain similarity with what you already have, I will not over modify your code, although it can be improved:

String unidades[] = { "uno", "dos", "tres", "cuatro", "cinco", "seis", "siete", "ocho", "nueve" };
String decenas[] = { "diez", "veinte", "treinta", "cuarenta", "cincuenta", "sesenta", "setenta", "ochenta", "noventa" };
String centenas[] = { "ciento", "docientos", "trecientos", "cuatrocientos", "quinientos", "seicientos", "setecientos", "ochocientos", "novecientos" };
String unidad_m[] = { "mil", "dosmil", "tresmil", "cuatromil", "cincomil", "seismil", "sietemil", "ochomil", "nuevemil" };
String decena_m[] = { "diez", "once", "doce", "trece", "catorce", "quince", "dieciseis", "dieciciete", "dieciocho", "diecinueve" };

System.out.println("escribe un numero");
int num = var.nextInt();
String numStr = Integer.toString(num);
for (int i = 0; i < numStr.length(); i++) {
    int aux = Integer.parseInt(numStr.substring(i, i + 1));
    if (aux == 0) continue;

    if ((numStr.length() - i) == 2) {
        if (aux == 1) {
            aux = Integer.parseInt(numStr.substring(i, i + 2));
            System.out.print(decena_m[aux - 10]);
            break;
        }
    }

    switch (numStr.length() - i - 1) {
    case 3:
        System.out.print(unidad_m[aux - 1]);
        break;
    case 2:
        System.out.print(centenas[aux - 1]);
        break;
    case 1:
        System.out.print(decenas[aux - 1]);
        break;
    case 0:
        System.out.print(unidades[aux - 1]);
        break;
    }
    System.out.print(" ");
}