First off, don't use SWIM. It doesn't provide legal protection, and the government doesn't seek out bluelight users anyway. DXM and Vyvanse isn't too dangerous, but you have to make sure it's only DXM. Most brands of DXM contain other drugs that don't contribute to the high and are very dangerous, some of which are more dangerous when taken with vyvanse. Look at the "active ingredients" section on the DXM, it'll tell you if it contains anything else. If it is pure DXM, the combination may be enjoyable (personal preference), but may not be. Whether you find it enjoyable or not though, it does increase the chance of a bad trip. Vyvanse can cause anxiety, which normally only happens at high doses, but when mixed with hallucinogens (including DXM) the anxiety can be more severe, and can turn into a bad trip. If it's pure DXM, the combo isn't a safety concern, but you might have a bad experience.
EDIT: I agree with the person below, DXM isn't a great drug to mix with others. If you're very expierenced with DXM and can handle it well, it's fun to mix with weed, but it intensifies the trip to the point where a new DXM user is bound to have a bad, or at least overwhelming trip. Mixing vyvanse with DXM isn't a long term or fatally dangerous safety concern, but mixing pretty much anything with DXM can cause or worsen a lot of side effects. With vyvanse, it won't cause severely dangerous side effects, but it can cause short term unpleasant ones. You'll probably throw up several times to say the least, and you'll be up all night and unable to sleep because of the Vyvanse, but you'll be misrebly tired and lazy once the DXM wears off, so youll end up just laying in bed with your eyes closed doing nothing but wishing you could sleep for hours. DXM often causes a unique hangover the next day as well, not like your typical headache and nausea, but you are extremely tired, lazy, and unmotivated. If you get 4 hours of sleep then have a DXM hangover the next day will be awful. Because it's safe, you're free to experiment with the combo, im just saying it probably won't be a great experience. If you wanna use both, id recommend taking the DXM alone, then taking the vyvanse the next day to counteract the hangover.