Илья Кочкин

думаю что тут проблема из за proxy

this.map = Vue.markRaw(new ymaps.Map(this.$refs.map, {
  ...

var1:1,
var2:2,
var3:3,
..
varN:N

form: {
var1:1,
var2:2,
var3:3,
..
varN:N
}

watch: {
form: {
handler: function(newForm, oldForm) {...}
deep: true
}
}

function getK() {
        $.ajax({
            url: '/index.php?route=create_order/OrderNew/getK',
            dataType: 'json',
            type: 'post',
            data: $('[name="time"').serialize(), //  возвращаемая k зависит от времени
            success: function (resp) {
              calc(resp);

            }
        });
    }

function calc(someValue){

return someValue*100;
}
getK();

document.addEventListener('click', function handler(e) {
    this.removeEventListener('click', handler);
});

function handler() {
  console.log('hello, world!!');
  this.removeEventListener('click', handler);
}

document.addEventListener('click', handler);

document.addEventListener('click', handler, { once: true });

let fn = function (e) {
  document.removeEventListener('click',fn);
}
document.addEventListener('click', fn);

в $header если сделать вардамп выводится следующее

string(324) "HTTP/1.1 200 OK Date: Tue, 11 Dec 2018 10:27:40 GMT Server: Apache X-Powered-By: PHP/5.6.28-pl0-gentoo Content-Length: 51408 Cache-Control: public Content-Transfer-Encoding: Binary Content-Disposition: attachment; filename=2018.12.07_[14:28:30]_+74562697525_103.mp3 Accept-Ranges: bytes Content-Type: audio/mpeg "

HTTP/1.1 200 OK
Date: Tue, 11 Dec 2018 10:27:40 GMT
Server: Apache
X-Powered-By: PHP/5.6.28-pl0-gentoo
Content-Length: 51408
Cache-Control: public
Content-Transfer-Encoding: Binary
Content-Disposition: attachment; filename=2018.12.07_[14:28:30]_+74562697525_103.mp3
Accept-Ranges: bytes
Content-Type: audio/mpeg

$header1=preg_replace('~^.+Content-Disposition: attachment; filename=(.+)Accept-Ranges.+$~is','$1',$header);

preg_replace('~^.+src="([^"]+)".+$~m',  '$1', $text);