It is quite acceptable to wear an argyll without a waistcoat and is generally worn unbuttoned. As you are wanting to honour the Scots attire properly, might I suggest wearing a tweed argyll with brass buckled belt, as the silver buttoned argyll that you are wearing is "formal day wear" and should not be worn with a tartan tie. There are those,me included, that consider a tartan tie as a no no, under any circumstances. I mean no disrespect to you and I hope you take my reply as being helpful rather than critical.