From Fairplay, (watch for speed traps) drive north on State highway 9 to a left (west) turn onto County Road 12. If coming from Breckenridge, drive south on State highway 9 across Hoosier Pass to Alma (watch for speed traps) and turn west onto either CR10 (which joins CR12 after a couple miles) or drive another mile south and turn west directly onto CR12. CR10 is passable for any passenger car, but is much slower, bumpier, and harder to find than CR12. The graded, gravel road up to the provided coordinates is suitable for passenger cars and can be driven all the way to where it crosses Mosquito Creek as the road begins a large turn to the south. If in a 2WD, low clearance vehicle, park in this vicinity and begin walking. There are several pullouts along this section. Beyond the creek crossing, the road becomes a rough, rocky 4WD trail; high clearance required.
NOTE: If you're unfamiliar with Mosquito Pass, the road can be driven in many stock SUVs equipped with 4x4 & higher clearance. However, the road is continuously rocky and does present some degree of clearance challenges for stock SUVs. It's the type of road that will put a fair amount of stress on your vehicle's joints & seals. If you're undecided, immediately after the indicated TH coordinates is a long, steep climb as the road winds south which will give you an idea of the road condition above this point. Snow will likely block passage on this side of the pass higher up on the 4x4 road later into the summer than on the west (Leadville) side—often until July 4th or later.
Some limited, primitive sites can be found along CR12, but watch for and respect Private Property signs.
If driving a 2WD or other vehicle that you highly value, it's best to park at the large curve mentioned in the trailhead description for Mosquito pass. High clearance 4x4 vehicles can continue on up a 4x4 track marked on various maps as either FR856 or FR783. This road is very rocky, very primitive, and definitely rough in places; recommended mainly for ardent 4-wheeling enthusiasts or those determined to shave off a few extra miles while possessing a generally cavalier attitude about the condition of their vehicle. The lower section is tightly wedged in between willows, so your turnaround options are limited until it begins switchbacking up the hill. We found one minor pull-off about 1/4 mile up the road that we elected to park at and began the hike from there.
NOTE: The lower section of this road is quite rocky and your options to avoid the larger rocks are often limited due to the encroaching willows. While it wouldn't necessarily require a modified vehicle to navigate this road, it would make the trip more pleasant. Once you reach the switchbacks, the road begins to show more of a dirt/rock mix. A few of the switchbacks are moderately loose and lighter trucks/SUVs may have issues getting enough traction if not weighted. Once above the switchbacks, the road turns into a more of a double-track with the tundra slowly attempting to reclaim portions of it. This is not a popular or well-driven road.
If you elected to park, hike up to the first switchback where a short spur road branches off. From here you have two choices. Your goal is to reach a saddle at 13,250' to the southwest of Mt. Tweto that connects with Treasurevault Mountain. To get there, you can either follow the road as it switchbacks and turns to the east/right (adding extra mileage), or you can take a more direct approach up the basin. The lower portion of the basin has quite a few willows and will likely be marshy in early summer or after sustained rains, but those obstacles can generally be avoided if a little care is taken. Choose your route and proceed up the basin, veering toward the west once the terrain below Treasurevault's E/NE slopes allows you to do so. As you approach the saddle, the terrain becomes more rocky.
From the saddle, proceed up the ridge until you see the faint beginnings of a use trail cutting over to the Arkansas/Tweto saddle. This 'trail' can be difficult to spot (and stay on), so if you don't find it, or loose it, just aim for the Tweto/Arkansas saddle and pick your way over. The slope as you traverse is on the verge of being steep enough to make the traverse across a pain, but the rock is just stable enough that this route is still likely to save you some time compared to summiting Tweto and descending the connecting ridge, even if you lose the trail.
From the Tweto/Arkansas saddle, pick your way up class 2 or 2+ terrain to a false summit on Arkansas' SE ridge. Follow this ridge and scamper the last 40' to the summit up larger rocks. The difficulty should not exceed class 2+ and any substantial exposure can be avoided.
To descend, retrace your steps to the Tweto/Arkansas saddle, then traverse back across Tweto and return to your car, or scamper up Mt. Tweto, then return to the Tweto/Treasurevault saddle and descend the same route you came up.