iI know those pre-vaccine nerves all too well! Try not to worry - you carefully thought out this decision and are not rushing into everything. The vast majority of kids who receive vaccines never even get a mild fever.
Be confident - try and keep your little one calm (since in my experience needles hurt more going into a tense arm or leg...I speak from experience. I'm a complete wimp) and then maybe indulge in a treat afterwards.
Fingers crossed here for an uneventful day in your house!!