Hi, Kathleen,

That appears to be hawkweed -- though not technically classified as invasive in Vermont except on our high peak alpine zones -- which can feel aggressive when we want our other plants to be thriving.

Hawkweed, Hieracium spp., is often a great indicator of what is happening in your soil, as it prefers to grow in lean (low organic matter) and very well drained soils.  I see you are located in Lamoille County, just like me!  I work on gardens professionally and travel around the county seeing various growing conditions.  
When we see hawkweed predominate in a lawn, that means soil fertility is low and most likely, your soil is on the sandy side.  Lamoille County was also in drought conditions from April through the late Autumn last year, so if you are not supplementing water to your lawn and you mow below 3-4" high, you may have seen that only the plants best adapted to these challenging conditions were thriving (or surviving).

While you await your soil test results, check out this article on drought and lawn from the U of Illinois Extension.  Also, take note of other potentially problematic variables related to your lawn:

• Soil compaction -- from foot traffic, recent or past construction, etc.
• Height of mowing
• Average amount of water applied
• Do you mulch chop your clippings or gather and remove?
• Do you aerate or dethatch your lawn to increase water absorption?
• Do you fertilize your lawn with a pelleted or organic-matter-based fertilizer (a compost of some kind)?

Also check out the Lawn Resources from the UVM Extension Master Gardeners.

In the end, I believe you will want to have a plan to improve soil conditions before you start removing the chickweed.  This will ensure that the bare patches of soil that remain won't just fill back up with other undesirable lawn plants.  

Check back in if you need help interpretting your test results and if you need help making a plan.

Cheers!
Genica