yes you can become member of A-kasse, if you have work as a "vikar". If you work less than 30 hours per week, you could consider only to pay for part-time membership. However we recommend you to ask the A-kasse about the consequences about part-time vs. full-time membership before you decide.
You must be aware that you need to meet the income-requirement, so if you have months with income less than DKK 19,448 then it will take longer than 1 year before you can apply for dagpenge.
Let me know if you have further questions.
Anders Weber, www.a-kasser.dk