You can certainly walk north of Vastenjaure as well via the bridges across Voujatädno at Sallohaure. There is no trail there, but that is also the case all the way throughout Sarek so you must know for certain how to use your maps. As long as you can identify the surrounding mountains for sure, navigation in these areas is generally easy. Padjelanta extends westwards all the way to the Norwegian border, but west of the large lakes there are very few people since it's so far from any trails or road access. Boats are available by the locals in the villages, so you just have to ask around. I have no idea about the cost and there may also be some times (during intensive reindeer herding periods) when they may not be very excited about spending time driving tourists, so the bridges may be a safer bet. Here: http://www.padjelanta.com/en/kommunikationer/sallojaure.asp you can find some phone numbers where you might get a better answer than I can provide.
I have only been in that western area in mid august, so I don't know how much water there is there in mid July. The Hurrejåhkå stream has a reputation of being problematic while there is still snow melting upstream. In Sarek along your proposed route the two streams that may be difficult are Njavvejågåsh right after Suorva (go upstream until it levels out) and Tjågnorisjågåsj just before Skarja (sometimes there is a snowbridge upstream, otherwise go down all the way to Rahpajåhkå). The last one is a glacier stream, so the water level is usually a lot lower early in the morning compared to late afternoon. Generally about the weather it's a big gamble. In particular since the Sarek area is, by far, the rainiest part of Sweden... last year I think I had both rain and sunshine every single day there!
/Pär