  State-of-the-art transmon qubits rely on large capacitors which
systematically improves their coherence due to reduced surface loss
participation. However, this approach increases both the footprint and the
parasitic cross-coupling and is ultimately limited by radiation losses - a
potential roadblock for scaling up quantum processors to millions of qubits. In
this work we present transmon qubits with sizes as low as 36$ \times $39$
\mu$m$^2$ with $\gtrsim$100 nm wide vacuum gap capacitors that are
micro-machined from commercial silicon-on-insulator wafers and shadow
evaporated with aluminum. After the release in HF vapor we achieve a vacuum
participation ratio up to 99.6\% in an in-plane design that is compatible with
standard coplanar circuits. Qubit relaxation time measurements for small gaps
with high vacuum electric fields of up to 22 V/m reveal a double exponential
decay indicating comparably strong coupling to long-lived two-level-systems
(TLS). The exceptionally high selectivity of $>$20 dB to the
superconductor-vacuum surface allows to precisely back out the
sub-single-photon dielectric loss tangent of aluminum oxide exposed to ambient
conditions. In terms of future scaling potential we achieve a qubit quality
factor by footprint area of $20 \mu \mathrm{s}^{-2}$, which is on par with the
highest $T_1$ devices relying on larger geometries and expected to improve
substantially for lower loss superconductors like NbTiN, TiN or Ta.
