Um, nmd was asking if it was safe to not take the Vyvanse every day, they were not asking if they should take extra. They wanted to know if they have to take it every day, even on the days they don't really feel they need it. Yes it should be safe to only take it as needed. There is no requirement to take it every day, or even to take your entire dose on the days you do take it. You could ask your doctor.
If you have been taking the 70mg every day for a long time, then you might notice some mental symptoms on days you don't take it, such as anxiety, depression, irritability and the return of the symptoms you were using it to treat. But it is not normally dangerous to not take it. It's actually good to not take stimulant meds on a daily basis and just take them as needed. Please note though that your tolerance to the medication may go down if you stop taking it for a while.